K20 pulling 19k lbs...

[Dano500]

Hey, guys and gals...

New to the site, but glad I found it. So, little bit of background, I got a heck of a deal on a fifth wheel, which is my first one, so I knew nothing about all of the weight ratings and towing requirements. lol
No regrets, but my current '98 K1500 can't do the job; 19k lbs. So, came across this 1984 K20 and want to create a tow monster out of it, but I have a few questions and I'd like to pick your brains.

72k original miles and all intact as of right now. Hadn't ran in a couple of years, but only because they left it sitting and just never messed with it again. I got it running again and still having problems I posted in tech section, but the main problem is that it is a 350 with 175hp & 275 tq. lol

Overall, body is in good shape. I see oxidation and surface rust, but further inspection makes me think media blasting can get rid of it, followed by some treatment. I do have to cut out some cancer from the door sills, but I think that's it so far. Good bed, cab corners, doors, fenders, but overall a decent survivor.

So, I figure I have to be around 800 ft lbs of torque to pull that trailer, so in order to get there the most affordable, I am going caddy 500 from a 71' and do a few upgrades with rebuild. I will also look for bigger brakes, but the trailer does have power brakes. I don't know if the one ton springs are different from 3/4, but I will look for stiffer springs and air bags. Still not sure of tranny/ gearing changes needed.

Question 1: Do you guys think it's possible?

Queston 2: Should I consider value of truck at all?


Thanks in advance,
-Dan

 

[HotWheelsBurban]

The Cadillac motor can be put in a square body, but it'd be easier and cheaper to go with a 454 and TH400 transmission. Since you have a K series 4wd that complicates things because you'll need a 4wd compatible transmission.
Back when these trucks were new, there was a shop in Houston, that bought a new C10/C15 Burb with a 454/400 combo (back in the mid 70s you could get them optioned that way....). He'd pull the engine, trans, driveshaft and rear axle and sell them (story was he had racers that would buy them). Then he'd install the complete driveline package from the previous year Cadillac, in the Burb. This shop primarily serviced Cadillac's and GM makes, so parts were available; besides getting whatever you needed from the dealership parts department.
We found out this, because we bought one of these Burbs in '79, it was a '75 GMC with '74 Cadillac parts. This is NOT a drop in swap!!!
You have to convert the rear axle to leaf springs/pads from the coil springs and 4 link setup the cars have (but this does get you the 12" Cadillac drum brakes which are bigger than the 11' the Burb had to begin with. Your K20 may have 13" drums from the factory though. Some welding has to be done to the engine cradle/crossmembers, and the motor/trans mounts are different. The standard Cadillac engine (472/500) uses a front sump oil pan; what you need to drop this engine where a Chevy used to be, is the rear sump pan that was only used on the '67-76 Eldorado. These are very rare and expensive now and NLA from GM for many years.
The stock 12 bolt truck rear axle should be sufficient for a stock engine, a 14 bolt semi floating axle would be better,and a 14 bolt full floating rear axle would be best. Cadillac engine bellhousing pattern is different than Chevy, so you'll need a "B-O-P" pattern (which also fits Cadillac engine, Chevys are the outlier here) transmission or an adapter plate.
In the late '90s and early 2000s the drag boat community discovered these 472/500 Cadillac engines and what massive torque makers they are. At that time there was some aftermarket speed equipment available for them; now there's virtually nothing. What exists primarily comes from a couple of companies and is expensive. Buddy of mine has a custom '61 Coupe DeVille with a built 500 in it; I know he spent a s**tload of money on this car and a LOT of it was in the power train.
Bottom line: it CAN be done, but it won't be quick and you'll need a big checkbook! And a big block Chevy will go in a LOT simpler and give you more bang for the buck.

 

[Grit dog]

First are you sure the 5ver is 19klbs? That would make it one of the very largest 5vers on the market. Even if it’s only say 16k dry with a 19kgvw.
Like, it’s gotta be a 44’ triple axle or similar to be that heavy.
To answer your question, yes anything is possible, however what you’re considering is not probable whatsoever, unless you’re just trying to build a truck that will pull it to the nearest state park in the middle of a flat, low altitude, fly-over state like Kansas.
You don’t need 800 ft lbs to pull that trailer but even with that much torque on tap, and an old 3-4speed trans of any flavor you’ll have more $ into the trans and gears than you’d like and it will be a 55MPH tops screamin at redline big block.
Back to the engine. Horrible choice. But arguably not much worse than building a 700hp Chevy big block or any other gasser. The chances it will be even remotely suitable for towing that load and not just obliterating tires will depend how well you can retrofit a semi truck radiator into a squarebody and actually pump enough water and air through it.

Chassis? Yeah sure maybe. The place I worked as a kid only broke full floating axles and frames occasionally on NEW trucks doing the same as you’re thinking. (Yes they’d hook a 20k trailer full of trees and equipment and drag it back, was good job security for me as the mechanic and as a bonus I learned how to repair truck frames that snapped…)
While on chassis. First of a long list of issues is your truck likely has a semi floated rear axle. Good luck there and can’t make it a dually.
Next I’m sure that crusty old spring mounts on a truck that has a fair amount of cancer from what you describe, will not be happy with your pin weight choices. Probably not if they were new either….
Brakes? Trailer has em so you’re good there, right? That’s just advertising that you’ve never towed a big trailer with an old truck, or more accurately never towed a big trailer at all. Break out your wallet again here if it hasn’t already smoldered into ashes from getting too hot with the credit cards.
Back to the old caddy motor. I’d do the engine last, and see how far you make it with the rest of an absolute franken truck before even worrying about what’s under the hood. It’s easier to drive it around the property for a couple years with the old 350 while doing EVERYTHING else and working that extra job to keep the credit cards below critical temperature. Plus when you’re done trying you won’t have $10-15k wrapped up in an engine that no one wants because it’s not a Chevy big block.
And if you do actually make it that far and completely re create the truck to be as capable as a new 1 ton diesel off the dealers lot, you’ll then realize that you need a diesel not a caddy engine and you’ll be ahead in that department.
Plus you could start saving and searching now for that 12V and a good 6 speed manual. It won’t be cheap but it will be easier and make more torque than you’d ever get out of a gas engine build.

So to answer your second question, no, the current value of the truck now does not mean anything. You’ll top that with just one of the multiple major rebuild, purchase, fab or magic projects towards building the truck.

 

[Nasty-LSX]

Bottom line: it CAN be done, but it won't be quick and you'll need a big checkbook! And a big block Chevy will go in a LOT simpler and give you more bang for the buck.

So true$$$$$$$ might as well buy a diesel truck

 

[Grit dog]

The Cadillac motor can be put in a square body, but it'd be easier and cheaper to go with a 454 and TH400 transmission. Since you have a K series 4wd that complicates things because you'll need a 4wd compatible transmission.
Back when these trucks were new, there was a shop in Houston, that bought a new C10/C15 Burb with a 454/400 combo (back in the mid 70s you could get them optioned that way....). He'd pull the engine, trans, driveshaft and rear axle and sell them (story was he had racers that would buy them). Then he'd install the complete driveline package from the previous year Cadillac, in the Burb. This shop primarily serviced Cadillac's and GM makes, so parts were available; besides getting whatever you needed from the dealership parts department.
We found out this, because we bought one of these Burbs in '79, it was a '75 GMC with '74 Cadillac parts. This is NOT a drop in swap!!!
You have to convert the rear axle to leaf springs/pads from the coil springs and 4 link setup the cars have (but this does get you the 12" Cadillac drum brakes which are bigger than the 11' the Burb had to begin with. Your K20 may have 13" drums from the factory though. Some welding has to be done to the engine cradle/crossmembers, and the motor/trans mounts are different. The standard Cadillac engine (472/500) uses a front sump oil pan; what you need to drop this engine where a Chevy used to be, is the rear sump pan that was only used on the '67-76 Eldorado. These are very rare and expensive now and NLA from GM for many years.
The stock 12 bolt truck rear axle should be sufficient for a stock engine, a 14 bolt semi floating axle would be better,and a 14 bolt full floating rear axle would be best. Cadillac engine bellhousing pattern is different than Chevy, so you'll need a "B-O-P" pattern (which also fits Cadillac engine, Chevys are the outlier here) transmission or an adapter plate.
In the late '90s and early 2000s the drag boat community discovered these 472/500 Cadillac engines and what massive torque makers they are. At that time there was some aftermarket speed equipment available for them; now there's virtually nothing. What exists primarily comes from a couple of companies and is expensive. Buddy of mine has a custom '61 Coupe DeVille with a built 500 in it; I know he spent a s**tload of money on this car and a LOT of it was in the power train.
Bottom line: it CAN be done, but it won't be quick and you'll need a big checkbook! And a big block Chevy will go in a LOT simpler and give you more bang for the buck.

And your bottom line is 100% correct but also still completely not feasible for the OPs scenario and we haven’t moved past just the engine and trans.

 

[HotWheelsBurban]

First are you sure the 5ver is 19klbs? That would make it one of the very largest 5vers on the market. Even if it’s only say 16k dry with a 19kgvw.
Like, it’s gotta be a 44’ triple axle or similar to be that heavy.
To answer your question, yes anything is possible, however what you’re considering is not probable whatsoever, unless you’re just trying to build a truck that will pull it to the nearest state park in the middle of a flat, low altitude, fly-over state like Kansas.
You don’t need 800 ft lbs to pull that trailer but even with that much torque on tap, and an old 3-4speed trans of any flavor you’ll have more $ into the trans and gears than you’d like and it will be a 55MPH tops screamin at redline big block.
Back to the engine. Horrible choice. But arguably not much worse than building a 700hp Chevy big block or any other gasser. The chances it will be even remotely suitable for towing that load and not just obliterating tires will depend how well you can retrofit a semi truck radiator into a squarebody and actually pump enough water and air through it.

Chassis? Yeah sure maybe. The place I worked as a kid only broke full floating axles and frames occasionally on NEW trucks doing the same as you’re thinking. (Yes they’d hook a 20k trailer full of trees and equipment and drag it back, was good job security for me as the mechanic and as a bonus I learned how to repair truck frames that snapped…)
While on chassis. First of a long list of issues is your truck likely has a semi floated rear axle. Good luck there and can’t make it a dually.
Next I’m sure that crusty old spring mounts on a truck that has a fair amount of cancer from what you describe, will not be happy with your pin weight choices. Probably not if they were new either….
Brakes? Trailer has em so you’re good there, right? That’s just advertising that you’ve never towed a big trailer with an old truck, or more accurately never towed a big trailer at all. Break out your wallet again here if it hasn’t already smoldered into ashes from getting too hot with the credit cards.
Back to the old caddy motor. I’d do the engine last, and see how far you make it with the rest of an absolute franken truck before even worrying about what’s under the hood. It’s easier to drive it around the property for a couple years with the old 350 while doing EVERYTHING else and working that extra job to keep the credit cards below critical temperature. Plus when you’re done trying you won’t have $10-15k wrapped up in an engine that no one wants because it’s not a Chevy big block.
And if you do actually make it that far and completely re create the truck to be as capable as a new 1 ton diesel off the dealers lot, you’ll then realize that you need a diesel not a caddy engine and you’ll be ahead in that department.
Plus you could start saving and searching now for that 12V and a good 6 speed manual. It won’t be cheap but it will be easier and make more torque than you’d ever get out of a gas engine build.

So to answer your second question, no, the current value of the truck now does not mean anything. You’ll top that with just one of the multiple major rebuild, purchase, fab or magic projects towards building the truck.

Yes if you just need mountain pulling torque, can't beat a 12 valve Cummins or a Duramax/Allison combo. These can be swapped in a square body truck too, but again it will not be cheap by any means.

 

[Frankenchevy]

I’m assuming this isn’t a fifth wheel travel trailer at 19k…

I don’t think camper companies would make such a thing for many reasons. The tongue weight of a modern 5’r that has a 15k Gvwr already exceeds the payload capacity of even a modern diesel. A new 4x4 crew cab diesel has a payload somewhere in the 4k range. To get a higher payload you need a 2wd, single cab, dually gasser.

Also, you’d have to assume that they’d assume most people are towing big 5’rs with a modern one ton. Add the Gvwr of the one ton to the Gvwr of the trailer and you are in Class A territory.

19k behind any truck is a bad idea for a novice tower, especially behind an older light duty pickup. If you are married to the 5’r and it is actually 19k, your money would be better spent on a medium duty truck ie 4500-6500 chassis nomenclature. Look at a ram 5500 or a topkick…

 

[Grit dog]

OP you still don’t know anything about weight ratings and towing requirements now either, or you wouldn’t even be considering this. Please take that to heart. It’s not a bad thing. We all don’t know schitt about some things but considering something like this also should tell yourself that you don’t know much about old cars/trucks/hot rods either. Or you’d have seen the futility in what you posted before even hitting the send button.
Save yourself the problems and failure and ditch the idea now. Fix up the old Chevy to drive around and go to the feed store and start picking out which diesel Duramax or Cummins factory powered truck you can afford (because no one can afFORD a FORD Flowerjoke) and learn what modifications you may/will need to do to a truck that is far newer, far safer, and far more capable in stock form.

 

[Redfish]

I am in agreement with @Grit dog I find it difficult to imagine a 19K fifth wheel travel trailer. Unless OP means an actual 5th wheel like an 18 wheeler...

My fifth wheel travel trailer is just under 11K dry, it's almost 38 feet long and my '19 Duramax definitely knows it's back there. I would not even consider trying to outfit a K20 for 19K pounds. Never. Nope. Not At All. You need dual rear wheels and a good chassis.

Too many decent used diesels out there to take chances with something like this. Best case scenario is you mechanically destroy your truck. Worst case is you destroy lives.

 

[Bennyt]

My '90 Squarebody CC DRW is rated to tow 13k. If I were to tow a trailer that weighed more than that regardless whether its 5th wheel, bumper, or gooseneck and get in an accident, I'd most likely be at fault regardless as I exceeded the weight ratings of the vehicle.

As far as a Cad 500 motor...I'm going against the consensus here but I'll give my opinion regardless as I am a fan of them.

I've met a few that have done it and been told it is almost a bolt in on a squarebody. Supposedly, all you have to do is swap the frame stands from side to side and make the appropriate crossmember mods. Haven't done it myself but have plans to build one one day for cruising as it is hard to beat the power and smoothness of a Cad 472/500. The few I've been around are as smooth as a new Lexus, and if you use aluminum heads and intake, are barely heavier than a SBC and they are fairly compact. Put a 2.73-3.5 gear and FI and you'll have power and MPG. I wouldn't swap my 454 for one but if i was starting with nothing, i'd consider it.

 

[Grit dog]

I am in agreement with @Grit dog I find it difficult to imagine a 19K fifth wheel travel trailer. Unless OP means an actual 5th wheel like an 18 wheeler...
Best case scenario is you mechanically destroy your truck. Worst case is you destroy lives.

And the most likely scenario is you end up with a built or half built BOP motor in the corner on a stand and a truck that is all torn apart and now no one wants 77% of a complete truck with 23% in boxes in the truck bed.

 

[Dano500]

The Cadillac motor can be put in a square body, but it'd be easier and cheaper to go with a 454 and TH400 transmission. Since you have a K series 4wd that complicates things because you'll need a 4wd compatible transmission.
Back when these trucks were new, there was a shop in Houston, that bought a new C10/C15 Burb with a 454/400 combo (back in the mid 70s you could get them optioned that way....). He'd pull the engine, trans, driveshaft and rear axle and sell them (story was he had racers that would buy them). Then he'd install the complete driveline package from the previous year Cadillac, in the Burb. This shop primarily serviced Cadillac's and GM makes, so parts were available; besides getting whatever you needed from the dealership parts department.
We found out this, because we bought one of these Burbs in '79, it was a '75 GMC with '74 Cadillac parts. This is NOT a drop in swap!!!
You have to convert the rear axle to leaf springs/pads from the coil springs and 4 link setup the cars have (but this does get you the 12" Cadillac drum brakes which are bigger than the 11' the Burb had to begin with. Your K20 may have 13" drums from the factory though. Some welding has to be done to the engine cradle/crossmembers, and the motor/trans mounts are different. The standard Cadillac engine (472/500) uses a front sump oil pan; what you need to drop this engine where a Chevy used to be, is the rear sump pan that was only used on the '67-76 Eldorado. These are very rare and expensive now and NLA from GM for many years.
The stock 12 bolt truck rear axle should be sufficient for a stock engine, a 14 bolt semi floating axle would be better,and a 14 bolt full floating rear axle would be best. Cadillac engine bellhousing pattern is different than Chevy, so you'll need a "B-O-P" pattern (which also fits Cadillac engine, Chevys are the outlier here) transmission or an adapter plate.
In the late '90s and early 2000s the drag boat community discovered these 472/500 Cadillac engines and what massive torque makers they are. At that time there was some aftermarket speed equipment available for them; now there's virtually nothing. What exists primarily comes from a couple of companies and is expensive. Buddy of mine has a custom '61 Coupe DeVille with a built 500 in it; I know he spent a s**tload of money on this car and a LOT of it was in the power train.
Bottom line: it CAN be done, but it won't be quick and you'll need a big checkbook! And a big block Chevy will go in a LOT simpler and give you more bang for the buck.

Thanks for the feedback, I am not dead set on the 500, so if I can get a 454, or pontiac 455 up to the torque I need, that's fine by me. I'm just researching it all now, so no big hurry. The dang thing isn't even running yet. lol

 

[Dano500]

First are you sure the 5ver is 19klbs? That would make it one of the very largest 5vers on the market. Even if it’s only say 16k dry with a 19kgvw.
Like, it’s gotta be a 44’ triple axle or similar to be that heavy.
To answer your question, yes anything is possible, however what you’re considering is not probable whatsoever, unless you’re just trying to build a truck that will pull it to the nearest state park in the middle of a flat, low altitude, fly-over state like Kansas.
You don’t need 800 ft lbs to pull that trailer but even with that much torque on tap, and an old 3-4speed trans of any flavor you’ll have more $ into the trans and gears than you’d like and it will be a 55MPH tops screamin at redline big block.
Back to the engine. Horrible choice. But arguably not much worse than building a 700hp Chevy big block or any other gasser. The chances it will be even remotely suitable for towing that load and not just obliterating tires will depend how well you can retrofit a semi truck radiator into a squarebody and actually pump enough water and air through it.

Chassis? Yeah sure maybe. The place I worked as a kid only broke full floating axles and frames occasionally on NEW trucks doing the same as you’re thinking. (Yes they’d hook a 20k trailer full of trees and equipment and drag it back, was good job security for me as the mechanic and as a bonus I learned how to repair truck frames that snapped…)
While on chassis. First of a long list of issues is your truck likely has a semi floated rear axle. Good luck there and can’t make it a dually.
Next I’m sure that crusty old spring mounts on a truck that has a fair amount of cancer from what you describe, will not be happy with your pin weight choices. Probably not if they were new either….
Brakes? Trailer has em so you’re good there, right? That’s just advertising that you’ve never towed a big trailer with an old truck, or more accurately never towed a big trailer at all. Break out your wallet again here if it hasn’t already smoldered into ashes from getting too hot with the credit cards.
Back to the old caddy motor. I’d do the engine last, and see how far you make it with the rest of an absolute franken truck before even worrying about what’s under the hood. It’s easier to drive it around the property for a couple years with the old 350 while doing EVERYTHING else and working that extra job to keep the credit cards below critical temperature. Plus when you’re done trying you won’t have $10-15k wrapped up in an engine that no one wants because it’s not a Chevy big block.
And if you do actually make it that far and completely re create the truck to be as capable as a new 1 ton diesel off the dealers lot, you’ll then realize that you need a diesel not a caddy engine and you’ll be ahead in that department.
Plus you could start saving and searching now for that 12V and a good 6 speed manual. It won’t be cheap but it will be easier and make more torque than you’d ever get out of a gas engine build.

So to answer your second question, no, the current value of the truck now does not mean anything. You’ll top that with just one of the multiple major rebuild, purchase, fab or magic projects towards building the truck.

Hello, seems like you are bashing more than giving advice, but I appreciate it anyway, all input is good input at this point. So the trailer is 18.9k gross weight and the tongue weight is 2.8k lbs to answer your questions on it.

I definitely wasn't planning on doing anything with old crusty parts as you put it. lol I definitely know I will be buying new and stronger parts, just looking into this as possible option to spending $60k on a truck I do not want to daily drive. The engine can be up to 500+ hp and 600+ torque for less than $3k. There are plenty of builds out there with this platform that use the caddy engine, so it's not like I'm flying blind and I'm not needing it like tomorrow. I'll definitely run it around for a while, and keep looking up the parts.

You are saying the trailer brakes are useless? I never said the current brakes will be fine, I definitely know I am going to be looking for bigger brakes . I do wonder about the frame like you mentioned and I don't know the differences yet between the 3/4 ton and one ton as far as thickness and strength, but that is definitely on my list along with the springs and bags concern.

IDK, just an idea at this point, but I'll keep looking into previous builds.


Thanks for the advice!
-Dan

 

[Dano500]

I’m assuming this isn’t a fifth wheel travel trailer at 19k…

I don’t think camper companies would make such a thing for many reasons. The tongue weight of a modern 5’r that has a 15k Gvwr already exceeds the payload capacity of even a modern diesel. A new 4x4 crew cab diesel has a payload somewhere in the 4k range. To get a higher payload you need a 2wd, single cab, dually gasser.

Also, you’d have to assume that they’d assume most people are towing big 5’rs with a modern one ton. Add the Gvwr of the one ton to the Gvwr of the trailer and you are in Class A territory.

19k behind any truck is a bad idea for a novice tower, especially behind an older light duty pickup. If you are married to the 5’r and it is actually 19k, your money would be better spent on a medium duty truck ie 4500-6500 chassis nomenclature. Look at a ram 5500 or a topkick…

Yup, it is a 5th wheel trailer. 15.9k dry weight & 18.9k gross weight, which includes 1k lbs of fresh water and 2k cargo. I was looking at a 3500 RAM before, but not interested in spending $60k.

I've seen this truck with 454 towing 14k lbs, which is what gave me the idea.

 

[Dano500]

OP you still don’t know anything about weight ratings and towing requirements now either, or you wouldn’t even be considering this. Please take that to heart. It’s not a bad thing. We all don’t know schitt about some things but considering something like this also should tell yourself that you don’t know much about old cars/trucks/hot rods either. Or you’d have seen the futility in what you posted before even hitting the send button.
Save yourself the problems and failure and ditch the idea now. Fix up the old Chevy to drive around and go to the feed store and start picking out which diesel Duramax or Cummins factory powered truck you can afford (because no one can afFORD a FORD Flowerjoke) and learn what modifications you may/will need to do to a truck that is far newer, far safer, and far more capable in stock form.

Thanks for the advice, Grit, I definitely appreciate it, but I still think most of it is possible. I've seen a k20 with a 454 pulling 14k lbs and it was oem. 4l80e will have overdrive and can be built to have plenty of towing power. My concern is more what I would need to do to address the stopping issue and frame strength and maybe it's not possible because of those two factors, but I aim to find out (before any build). We shall see...