There are no 10 x 2.5 SBP backing plates. Original A-Body SBP rear 10 in brakes are 10.75. Original 73 up BBP 10 in A-Body rear brakes are 2.5. The backing plates are different than the SBP backing plates. But the 10 x 2.5 brakes came on most other platforms including PU Trucks.
66fs mistyped his reply... he meant to say, SBP 10in rear brakes are 10 X 1.75". That is, 1-3/4 inch wide, not 2-1/2 inch wide. The width is measured on the shoe, not the full depth of the drum, FYI.