What to do now? It's not an a-body rear end.

cuda dan said
I believe all 8 3/4 A body rear ends have the small 4" BC.
the B and E body 8 3/4 rears I've work on axles extend further from the tube end, but they also had wider brake drums.
they also have the larger 4 1/2" BC.
I installed a B body, narrowed 8 3/4 rear end in my 65. Updated the front end to 74 A body discs to get the larger 4 1/2 BC.

Your correct. The B and E brakes use different backing plates to match the axle offset and wider drums. But it's best to verify. These parts have been around along time and could have redrilled or mismatched parts.