The easiest would be to find the 4.10 3rd member from a '68-'72 3/4 ton, as it bolts right into the stock rearend housing. Unfortunatly, those are difficult to come by. If you don't mind doing the fab work (manly welding on new spring seats), a rearend from an '80s Dodge/Ford 3/4 ton van (Dana 60) will fit just fine. A 3.55 ratio is common in those. You'll want to watch out, though, some of those are semi float instead of full float like the original axle. For somehting that's just going to be a driver, semifloat will be fine. But if you plant on loading it, a full floater is better.
