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).
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Thanks for all the terrific info! I did get my axles from Dr. Diff. It was a couple months ago. I remember I told him I was switching from 7 1/4 sbp to 8 3/4 bbp but we didnt talk about the brakes... So I hope I got the right length for what I'm doing. I decided the best way to do this is to switch to the 10" with 2.5" linings. It means finding the other backing plates you mentioned, and other matching brake parts. It's kind of a pain, but it will give me the best braking power in the end.