So I bought an 8.75" B body housing, axles and perches for $200...

Kern Dog said

The 11” rear brakes are overkill unless you’re building a pickup truck to do hauling or towing. The extra weight and brake force are not a good match for even a car with 11” front discs. It is odd how the Dana equipped B and E body cars meant 11” drum brakes even with 11” front drums or discs.
Unrelated to the current topic but hey…I probably have A.D.D. So remember, it isn’t nice to pick on the disabled.

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I added 11x2.5" drum brakes to my Challenger when I was running 11.75" mopar disks up front, that combination produced well balanced braking that was better than it had when it was set up with 10.95" disks up front and 10x2.5" drums out back.

Did the same thing with my Duster which started out with the '73+ 10.95" factory BBP disks and 10x2.5" rear drums, went up to 11.75" disks in the front and 11x2.5" drums in the back. Great improvement in braking and not at all overkill for the street. More stopping power is better.

And for the OP, what you need to do to that B-body 8 3/4 will depend a lot on 1. what year it is, since B body rear axles are not all the same width and 2. what you plan on running for wheels. If you're staying with 14" or 15" wheels you'll likely need to shorten it. If you're going 17" or 18" wheels and modern tires, probably no shortening needed.