1972 would be the latest chassis that would keep the wheelbase corrct. Anything later has a longer wheelbase.
I agree with the others, swapping out the crossmember on the existing frame will get you the disk brake/power steering upgrade, it isn't any bigger or smaller than the original, and out back it would be simply a case of replacing the coils with the bags. Since the cotrol arms connect forward of the axle centerline, notching the chassis shouldn't be as bad, no sping mounts to keep in line, and you wouldn't have to convert anything to make it work. One more thing, the rear suspension on the '60-'72 Chevy truck is identical to the suspension that's under every Nextel, Busch, and Craftsman truck Nascar racers. It'll go very low. Swapping the body will require fabricating all of the mounts, especially if you go with the '80s chassis, on top of the mods for the bags.
