10 X 1-3/4” rear brake backing plates wanted

Wouldn't backing plate be the same for big and small bolt pattern since it is held on by bearing retainer?
No. The 10x1 3/4” small bolt pattern backing plates are exclusively unique to the A body 8 3/4” axle only. They have a unique offset for the small bolt axles and will not work with any other axle, nor will any other backing plates work with them. However, you can make the 7 1/4” 10” backing plates work if you drill the 5th hole to mount them.