What are your plans for wheels/tires?
I ask because you may not want to use A-body 8 3/4 specs. DoctorDiff can sell you custom length axles, the price difference is pretty negligible. On a Duster the factory rear track is substantially narrower than the front track, and depending on your wheel choice it may be better not to run the factory width.
Since you’re moving the spring perches already, you may also look into DoctorDiff’s 1/2” spring offset, which will buy you some extra space for wider tires.
Not saying to have to do either, but just considerations. I run a 68-70 B body 8 3/4 in my Duster at the B-body width, with a 1/2” spring offset and 295/40/18’s on 18x10’s.
