[66Submarine]

Well, I have no idea what would be closest swap as far as a donor, but the full-float Dana 60 rear and the 14 bolt GM are both super common and have great aftermarket support. The problem is that those are also going to be eight lug axles, which means you'll probably want/need to swap the front along with it.

You'll obviously also need to find something that has the same gear ratio if you keep the front, but you also might want to just look at better semi-float axles which will have the six lug pattern, or could be re drilled for it.