Two options.
Opt. #1: Pull the drive train, and replace it with the 390, 727, 8-3/4.
Opt. #2: Stay with the slant, and 904 ( beef both up a little). swap the 8-3/4 in, and redo the slant with Centrifugal supercharger, fuel injection.
Ok. My bad.
I've done the Moser axle replacement on one 8 3/4 which got me the big bolt pattern and wider rear because the brake shoes were then upgraded to 2 1/4 "modern" ones.
But the car already had big bolt patter front disk.
I've got another 8 3/4 housing on stand by in case the 8 1/4 in another A body of mine ever goes south or something. (Duster quarters need the wider stuff I think.)
I just assumed you were going to go with large bolt pattern.
But it sounds like you may be keeping the small bolt pattern rear which IIRC all A body 8 3/4s were.
Good deal.