B body 8 3/4 in Demon

Well, first, You are in for some fun.
This question has no answer, yet.
Is the B-axle stock width? What year B was it in?Or better is,what is the drum to drum distance?
The backspace Will depend on the width of the wheel, which will depend on the tire selected. You will also need to select the side of your car which has the smallest distance from the inner tub to the wheelhouse lip, as the rear ends are rarely centered.
The best idea is to map it all out on a strip of plywood or posterboard, using plumb-bobs to locate the various points of interference.
The car has to be jacked up under the dif, and be reasonably level from side to side. The map strip needs to be fixed to the floor so it cannot move, or indexed such that if it does, its easy to relocate.
After that its easy to find the maximum tub space, then what tire fits, then what rim-width works, and finally the backspace.
Any other way,is pretty much guessing.Unless someone else having the exact same combo, pipes up with what worked for him.
And finally a tip; If you're going for the max tire width, its best to make the bs slightly too deep, than the other way around.If it all ends up too wide, you're in trouble.But if its slightly narrow you can add a spacer between the wheel and the drum.
Work slowly and carefully. Good luck!