[oldusafmsgt]

I've got a 60 and a 66 with big blocks. It is no more involved than swapping from a 6 cylinder to a small block. There is only a 100 pound difference in weight between an iron headed BBC and a small block V8, and an original 63 GMC V6 is only 40 pounds lighter than a BBC. There is no need to swap the front suspension cross member, (especially on a 63-66) and a TH400/475 will clear a low trans tunnel floor. 68-72 BBC motor mounts, or aftermarket mounts (CPP) is all you really need, along with the driveshaft shortened slightly. It's an easy, common swap that people have been doing since these trucks were new.