76 breaks problem

I have a '76 4 door with the slant 6. Discs on the front and 10" drums with 1 3/4 shows on the back. I have read some threads about the centric 123.63044 being the right replacement drum. I ordered 2 from rockauto, website said 10" 1 3/4 but received 12".(returned) Went to ebay right part number but also received 12" drums. Any suggestions? Mechanic says its supposed to have 10" 2.5 shoes.
I cant seem to find 10" 1 3/4 drums anywhere. Should I look into swapping the backing plate to take the 10" 2.5 that "its supposed to have"?
Thanks guys

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