1970 duster slant six 8.25 rear swap

A 73-76 A Body 8.25 is pretty much a direct swap. Use the 8.25 shock plates. You will have to shorten the driveshaft a couple of inches.

An 8.25 from any other platform is going to be too wide. Yes they can be narrowed and replacement axles sourced - expensive! Also don't forget that the 8.25 is a LBP rear end so you may want to upgrade your front brakes to the 73-76 A Body disc brakes