8 3/4 Backing plates SBP question

News to me... A few years ago I was assured the 7 1/4 10x1-3/4 brakes were a direct swap to an 8 3/4.

72 back won't fit, different axle flange size and bolt pattern.
73-76 A body I believe fit. Still used the 10x1.75 brakes but used common axle flange pattern.
76 up for sure. Just tore another late 70s 7.25 apart last week. Direct fit.
But those are 10x2.5 shoes for 76 up.