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
Re: brakes difference 7-1/4 & 8-3/4 axles - 02/26/1903:33 AM
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.
Re: brakes difference 7-1/4 & 8-3/4 axles - 02/26/1904:45 AM
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.