Welcome to the site. Take your time, do your research, and fix it up. Not everything needs a LS swap. You can find a mildly built 350 for near the same price as a stock one.
I'm sure there are a lot of small things that need to be changed. Fuel system, engine mounting, transmission mounting, wiring, etc.
Engine and part of the cluster harness will be the big wiring parts.
I would go with the 78 because of the high hump. How often are you going to be looking under the seat?
Ona side note: before you place matting on the dr. side, line up all the holes and cut them out before you have a bunch of slices looking for bolt holes.
Before your blast LMC or any other manufacturer about how off the color is. Remember, your truck is 45 years old. Lots of sun damage over the years. Grant it we live in the PNW and have less sunny days than cloudy days. But interiors do fade.
You can see on the door panel how different...
Sounds like a switching valve. you should be able to apply 12v to power pin and here it click.
https://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/
Just hope it's not the problem I had which was several. Bad switching valve, dirty tanks, clogged socks, and degraded feed lines.
Take a look at your rear brakes. You could have leaking axle seals.
Generally, brake controllers don't control the truck brakes, they are just put in line to know when pressure is applied to apply the brakes on the trailer with an electric signal.
There are 2 types of differentials; "open" and "with traction device". An "open" differential will power only one wheel, if that tire looses traction, then you are not going anywhere. i.e. if you lift the tire off the ground offroading then you have no power to any wheels on that axle. A...
NP203 Full-time t/cases are just as strong as the NP205 Part-time t/cases with one difference. The Chain. Whereas the 205 is all gear driven the 203 is has the chain instead of gears to run power to the front drive. The chains are very heavy but will stretch over time. The 203s are heavier...
Welcome to the site.
What did you gap the plugs at? What did the distributor look like when you had the rotor off? Was there any play in the in the shaft? Instead of renting a timing gun, just buy one. If you are going to own older vehicles it will be handy to have one.
Check the fuel...