Need New Drums

The 10 “ x 2 1/2 “ Rear Brake Drum —- IS —- a 5 x 4 1/2…
The 10 “ x 1 3/4 “ Rear Brake Drum —- IS —- a 5 x 4 …..

Not the whole story. There are aftermarket axles out there that have the BBP but the SBP axle flange offset and require redrilling the SBP drums.

Which means, just because the axles measure 5x4.5” doesn’t mean that the backing plates he has are for the 10x2.5” brakes. It looks like the OP has backing plates and not drums, which is why he was measuring backing plates to get the drum diameter

Since it’s a narrowed B-body rear axle the plates are most likely for the 10x2.5’s, but you can’t tell that just from the axle bolt pattern.