Swap out 7 1/4 RE for an 8 1/4 RE

I would like to find out which entire rear end and axle unit would be the easiest and require the least modification to swap in my son's 76 Duster. The 76 had the larger bolt pattern and I'm trying to apply the KISS strategy to this swap over. Is there a bolt out(of a salvage car) bolt in(to the 76 Duster) rear axle out there and what car would it come out of. It wouldn't have to be a perfect swap, relocating lspring brackets and the like is do-able I just want the right span for the total unit with the proper bolt pattern. Any ideas? This is to be a street legal driver.