B body rear in an A body
I just put a 8 3/4 from a 66 b body in my 71 duster. I had the spring perchs moved in 1/2 inch ($65) on both sides. The 65 - 67 B bodies 8 3/4 are about 1/2 shorter on each side then the 68-70. I'm running the 15/7 factory steel rim with p225 70 and I have another inch and half space towards the outside and a ton of room on the inside if I want to run more of backspacing.
I think the earlier 8 3/4 look better on the dusters and demons then the factory rear.
Plus you get the big bolt axles and bigger brakes.