[Dieselwrencher]

Nice project! I think the only thing I did on a square body low hump floor was roll the lip of the firewall so a 4" down pipe would clear and so the dipstick would pass by. That was a C20 also and the frame was a little taller but it all worked out. That was the only 2wd square body I have done. I used a champion radiator and a 1st gen dodge intercooler that I had one side tank widened to fit around the chevy champion aluminum radiator. I did a 4x4 89 gmc burb the same way and they worked well. The 1st gen intercooler piping is smaller but nothing some chop cut rebuild won't fix.

That 518 is also a little closer to the engine than a 47re or rh so keep that in mind if you ever plan to swap to one. The difference is in the engine adapter plate. The 518s are non lock up so the converter is shallower. The engine plate is only 1" thick where the 47rh and re engine plates are almost 1.5" thick/deep. This puts the trans farther back and can get right into the firewall. Just something to think about. 518s are great transmissions. The non lock up thing just gets old. But if you never have it you don't know what you're missing.