Could a person use the backing plates and 10 x 2" brakes from a 7-1/4" rear to swap to a 8-3/4" all from E-body cars??

When I swapped an 1966 8.75" housing into my car the brakes from the 7.25" it was replacing all transferred over. Didn't even need to change backing plates.

You might measure/compare (between the two) the shoe width area in the drums & the shoe width itself & the "offset" of the b plates. EDIT & axle flange distance out from b plate

7 1/4 backing plates have 4 holes where the 8 3/4 plates have 5 holes.

I ain't eyeballed em but would it be possible to drill the 5th hole in the retainer? (assuming everything else matched).

I replaced the 7 1/4 rear in an 82 Cordoba with an 8 3/4 and the brakes swapped right over.

Did same here with an 80 F body. Somewhere along the 7 1/4 line, dunno what year, the ends of housing changed to accept big hardware and 5 bolt retainers.

My experience is that BBP brakes will interchange. E-body should be BBP.

the four holes line up, the fifth hole is redundant.

All 5 x 4.5" bolt pattern 7.25" rears have the same brakes as their 8.25", 8.75", 9.25" and 9.75" counterparts. The 5th hole in the backing plate is hidden behind the housing end flange.