Rear diff question

Yeah I wouldn't do all that work for a Ford 8". A Ford 9" or 8.8 maybe. You mean the 8" housing is 4" narrower? I wouldn't do that either unless you're planning a mini-tub, you'd need negative offset wheels to keep the tires off the springs if the backspace has to change by 2" per side.

I'd be looking for a 8.25", or even an 8 3/4 out of a 65-67 or 68-70 B body. Or yeah, a Ford 8.8 or 9". You're going to have to relocate perches and shorten the driveshaft anyway, might as well do it for something worthwhile.