What is the deal? SBP 10"×1 3/4" backing plates.

Nope, they've always been different and didn't interchange. The only other backing plates for the 8 3/4 with 10 X 1 3/4" brakes is the mid-60's B-body 6 cyl cars with a 7 1/4. They used the same brakes (except for the drums) as the A-body 8 3/4.