What's a good rear end to put in my Duster?

65-66 Fury or 68-70 B-body rears are a good replacement but you must cut the welds on the spring feet and move them in about 1/2 to 3/4 inch each side. This will also allow you to upgrade to 11" brakes. The flange to flange width increases about 2" but wheel choice can fix most issues there. You can also just use an 8 1/4 from another later model A body.