A Body swap with E body 8 3/4 with 4link

Professor Fate, nailed it.

What you have is a truck rear. The 8 3/4 center section is commonly referred to as a 741 it is considered to be the weakest (not judging, just stating) as the pinion shaft diameter is the smallest of the 3 versions (741,742,489), also the new gear selection for the 741 is very limited, 742 more options, 489 even more options.

12" brakes are huge for an A Body, also are you measuring the ID of the drum for that measurement?

Good news is that a truck axle is a good narrowing candidate.