I took the easy way out and got with a guy that had parted out a bunch of big block trucks. One stop shop for oil pan, dip stick, motor mounts, exhaust manifolds, big block 4wd trans - everything ya need to put that cheap C-body donor engine in yer truck. I did spend some time tracking down the correct power steering brackets and water pump pulley. As for it fitting, I'll get back to ya but looks bolt in to me. When they disco'd the big block in 78, the didn't bother changing anything about else about the truck. The front cross member still has 2 sets of motor mounts holes - 1 small block, 1 big block.

Valve cover clearance is an issue. The factory installed the smaller diameter dual diaphragm booster (200 and up) when the RB was ordered. Some say the engine will fit around the larger diameter single diaphragm booster, but ya just can't take the valve covers off. The C-body ya jack the RB from will likely have the smaller diameter booster but haven't tried that transplant and they predate aluminum masters so it's gonna want a 4 bolt master. I recently scored a 2 bolt one from a roached plow truck, fingers crossed. If you put on any kind of taller than stock fancy valve covers, they'll interfere with the heater box but there's room to notch it.

Coming soon to an RC near me: