[QUOTE=speedydeedy]To the best of my memory any Chevy or GMC 1/2 ton 1971 and 1972 you can just swap the axles and brake drums into your rear end housing for a 5 on 5" lug pattern.QUOTE]
That's a little confusing perhaps;
The 64-69 Chevy pickup 1/2 ton rears are the "narrow" rears. The 5 lug axle shafts from a 1/2 ton 71-up won't fit 64-69 due to the fact that they are longer shafts.
The only 6 lug 1/2 ton rear that can be swapped to those later axles is the 1970 rear. !970 is when the axles got longer, but still had 6 lug. The next year(71) chevy went to discs, & 5-lugs.
Notice I refer to "chevys" when I list axle shaft swaps. GMC's used a different differential manufacturer (if I recall correctly), at least on the 4x4 1/2 tons.
So, I think it would be best to grab a chev 1/2 ton rear assembly from a later 70's & up. Those later 70s saw some better gear ratios for better highway use. I have a 2.76 1979 rear in my 63 suburban. Good old GM interchanges; I wanted to stay with the 6 lugs, so I just went with a 4x4 rear which still used 6. If I wanted the 5 lug, a 2wd would solve that.