swapping 8.75 for 7.25, brake drum problem

Which axles did you use to go big bolt pattern?

SBP axles also have a different length (offset) to use the SBP 10x1.75" rear brakes.

10x2.5" backing plates will not interchange with 10x1.75" backing plates, as you've found out the offset is different. Further, if you had your SBP axles redrilled for BBP, you have to do the same with the brakes, because 10x2.5" brakes will not fit (even with the correct backing plates) because of the axle length.

So, if you want to go to BBP 10x2.5" brakes, you need to have:

1. The correct length BBP axles (not re-drilled SBP axles)
2. 10x2.5" rear brakes, with 10x2.5" backing plates, drums etc.

10x1.75" SBP brakes only work with SBP axles and backing plates. If you redrill a set of SBP axles to BBP, you have to redrill your 10x1.75" drums too. And, some aftermarket BBP axles are set up to use the stock 10x1.75" brakes with re-drilled drums. So, make sure your axles are the right length (Dr. Diff sells the correct lengths). And of course the same is true in reverse, if your axles were set up for 10x2.5" BBP brakes, you can't use the 10x1.75" brakes, even if you have the drums redrilled to BBP, because the axle length will be wrong.

As far as the BBP 10x2.5" brakes, they're pretty much all the same. They were used up until 1989 on Diplomats and 5th avenue's, so you don't have to find a 70's car. On early 80's vehicles you can find them on just about anything, including small trucks as the lighter duty trucks had the same bolt pattern. And any BBP set up should work, whether its attached to a 7.25, 8.25, 8.75, 9.25 or Dana rear axle. Some of the 8.25" rears only used 4 studs, so you may have to drill a hole in the backing plate for the 5th stud.