What Am I Doing Wrong????

I'm going to agree with the answers of the rear not being centered factory. The rears were offset. But one of the things you pointed out was the 8.75 came out of another A-bod. That doesn't necessarily equate direct bolt-in. The Duster here originally had a 7.25. We came across an 8.75 out of a '68 Dart that a buddy of ours was scrapping. $50! Jackpot!
Got it home, unbolted the 7.25 and started measuring: the spring perch location was different between the two axles and the perches needed to be moved a hair.