What cars have the same rear as a Duster?

Since you've already converted the front brakes to disc and LBP, your best bet would be from a 73-76 A Body either a 7.25 that is already LBP or better since you're going to convert to a V8 would be an 8.25. Both would be a bolt in; however you'd need the shock plates that come with the 8.25 as well as the 3 inch U Bolts