BBP Rear Brake Backing Plates for 1972 8.75

Wait, how will later 7.25 BBP backing plates work on any 8.75 housing? Even if the 7.25 is BBP, it's still a smaller diameter axle housing tube, right? And this is assuming 10" brakes across the board. Obvously backing plates from 9" 7.25 BBP brakes won't fit.