11 Inch Rear Drums?

I have a 71 Duster 225 slant six with a 73-76 front disc brake disc conversion and the rear has been converted to the large bolt pattern. I was under the impression that the rear drums would either be 9 or 10 inch, but upon inspection it turns out they are 11 x 2 1/2. My question is what vehicle might the rear brakes be out of so that I know what parts to order. The rear end was also swapped out for a 8 1/4 even though slant sixes came with 7 1/4. Thanks.

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