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

Correct. Total and complete incorrect internet garbage nonsense. They are totally different from the 8 1/4, 8 3/4 and Dana patterns.
How about a truck 9 1/4 thrown into that mix? Say, taking the brakes, backing plates and all and drilling the 5th hole to mount them on 8 3/4?