8 3/4 brake problem

Changing from 7 1/4 to 8 3/4 SBP. Tried to put my new drums on but won't fit, not big enough to clear post at top of backing plate. I thought the backing plate I bought was for 9 by 2 brakes, apparently not. It looks like I need 10 inch drum, so the question is can I buy 10 inch that will fit my SBP axles.