What did you use for the front conversion? The bolt pattern size will dictate what rear you will want to use to match. Truck and passenger 5 lug are different sizes, 5x5 on truck, 5x4.75 on GM car, 5x4.50 if you used ford or MoPar parts for the conversion. That can be an issue with getting matching wheels, as most wheels for the truck pattern are not available in the passenger patterns. There could also be an issue with a spare, depending on whic patter(s) you have.
That said, the '71-'72 rear would be the closest bolt in, and even then there's some slight alterations that would need to be made. I know that the Panhard bar mounts differently, and I seem to recall that there is a slight difference in how the trailing arms mount.
One more thing, if you have a 3.08 rearend gear, it is not a stock diff. In '60, the only gear ratios were 3.38, 3.90 (most likely what it is), and 4.10 in the 1/2 ton. The basic diff is the same as the '55-'59 trucks, and was used to '62. '63 saw the change to the 12 truck diff, used to the mid '70s. You could actually use a '65-'66 as a bolt in replacement, then use a shortened axle shafts from a '71-'76 2wd to get 5 lug.
