8 3/4 backing plates onto a small bolt with upgraded big bolt shafts

There is nothing to decide about the brakes. You said you ordered from Doctor Diff.
This is from the axle product page on his website:
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You need different (B/C/E etc.) backing plates, hardware and drums as I noted above. Get them as a set, there were some different widths over the years and each width used it's own backing plate and drum. NO mixing and matching.