I would shy away from a small block 400 in a application like this. Finding a good solid block when the motor has now been out of production for 30 years could be difficult, and overheating was a problem with those motors when they were new.
You've described the truck and the transmission, but said nothing of desired usage and performance. A 350 built for low end torque might be perfect, but a stroker 383 will be better. Dave's suggestion is a good one.
But if you want a strong performer while loaded you want a bigger motor. A GMC 450 is a big truck and pretty heavy. I'd be thinking 454 big block or maybe one of the bigger stroked versions of that motor if you want hot rod performance.
Where Ford seemed to design a new bellhousing pattern for every series of motors, Chevy motors are far more interchangeable. Your adapter will bolt to any Chevy V-8 made starting in 1955 up to sometime in the 90's. I don't know the details of the most recent Chevy motors. For about 40 years they shared bellhousing patterns, and starting in '58 those motors all shared the same motor mounts.
