Hello! I've been snooping around here for a while, but finally registered to start contributing.
I did almost this same swap quite a few years ago. Overall, it wasn't too involved.
Mine was a 78 C10 and it is now on an 88-94 K1500 chassis. I am using the later drivetrain which is a 350 TBI, 5 speed, manual t-case and then the 3.42 geared front and rear diffs. The rear diff is actually out of a GMT800 with rear disks, but that doesn't really matter for this discussion.
The TBI meant that the engine harness was not really integrated into the truck electronics, made things easier.
Body mounts - the front cab mounts are set on bushings that rest right on the frame, added a mounting pad and the frame width is perfect for this. The rear cab mounts needed fabricated. The core support mounts also needed fabricated, but the frame width is close here. The bed mounts got tabs off the frame on the rear 4 bolts and the front 4 bolts needed cross bars and tabs for the bolts since the newer frame dropped down under the cab. To get the body to be lower and not look so out of place, the fuel tank cross member needed to be removed and replaced with a flat one that was level with the top of the frame.
The radiator, brake booster, etc. under the hood more or less bolted right up.
The steering took an adapter from the 78 column to the newer steering box. I did use a column from a later model square so that I could get the tilt, intermittent wipers and the cruise control on the stalk. Also integrated the safety for the key in use with the 5 speed.
Inside the truck, the factory gauges work, the speedometer is the only one that needed help. I am running a box that converts the electronic speed signal into a cable driven action to drive the speedometer. There may be options from later square body trucks (think 3/4 or 1 ton trucks from say 90 or 91) that use the TBI engine and an electronic speed signal. I did not think about this at the time, plus I don't think the gauge font would match the other gauges.
Just add holes for the manual shifter of the transmission and t-case and used the boot from the newer truck on the transmission and a mount and housing from a square body on the t-case. I turned it around so I had to change the labels so that they are correct. Also, the shift lever needed tweaked so it wouldn't hit the seat in 4 hi.
For the clutch - I used a hydraulic master cylinder from an 87 C10 and got pedals from the same. Originally tried pedals from my year of truck (78) but never got the mechanical pedal to work with a hydraulic clutch so I switched masters. Works great now. The mounting pattern is available online. Add brackets to the firewall and column mount for the neutral safety and the cruise control cut off when the clutch pedal is moved.
The rear bumper just took swapping the frame mounting brackets from side to side to account for the frame width difference, but the front bumper did take fabricating new mounts.
Interior and lighting all use the 78 wiring harness and switches. The engine is a stand alone system and there is no trans module or BCM.
The biggest issue I have had over the years since I did this is picking out rims that I like. I really like the original rally style, but those are 15s and won't clear the rear disks. So I have been experimenting with GMT400 and GMT800 aluminum rims trying to get a look I like.
Below are a few photos (at least I hope they load) to show a couple of things mentioned above.
This is the front cab mount put into the frame. At this point, the width is perfect.
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Fuel tank cross member change and forward running bars for bed mount.
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Interior
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Current state, spring of 2024
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Good luck!!!!
