Lol, it is doable in that body to go B-body 8.75 rear, just moving perches. Not uncommon. 2nd, there were & are literally THOUSANDS of A-body Mopars with the '73 & up discs paired with a factory SBP 8.75 rear axles. Big'nLittles ain't hard to find.
I wondered about the B-Body axle and looked at the pictures again to see if the car had the “elephant on roller skates” look the factory cars had to try and see if I could tell if it was an A-Body or B-Body axle. But it looks to have widened wheels in the back so couldn’t tell.
Pretty sure it isn’t a SBP axle though as (I think) the center caps on the Rallye wheels are BBP only.
My best guess is - factory A-Body 8.75” with re-drilled axles and drums. Seems like a pretty popular option way back when for awhile, got a friend that did it to his car years ago, and currently has a BB Dart project in full swing that he bought with an axle it was done to when he got it.
The better option (and another idea) is the Moser resplined BBP axle swap. That was also pretty popular back when I would bet this car was actually built.
