My experience with the GMC's (this is 60-66 models) is that if you don't get into the big unique GMC V-6 motors, there isn't that much difference between Chevy's and GMCs, other than grills/trim. On my son's 66, the GMC's came with Hotchkiss (leaf spring) rear suspensions and the Chevy's with coil spring/track arm (NASCAR), but I think you could order either one with the other. I thinik by 68 that was no longer a difference as they both went leaf spring.
So basically if you stay away from the V6 big block, it is more of a cosmetic thing as far as I am concerned. The V6 is a cool motor, but surprisingly hard to find parts for them.
Having said the above, I am not an expert, but others on this board are.
