'60-'72 are for the most part coil spring, though some do have leaf spring, mostly 1 ton and 3/4 ton Camper Specials. The reason that you'll want to look for a late ('67-'72) rear is that they are more likely to have the 4.10 ratio. Most of the earlier diffs are either 4.56 or 5.13, depending on wheather they were 3/4 or 1 ton.
InMy40s-the diff pic you have linked in is a Chevy diff, most GMCs from '60-'72 use Spicer 60 or 70 diffs. There is also little difference between the HO56 and the HO72. Mainly it's just the original application, the 56 was used in 3/4, the 72 was 1 ton.
