rear brake question

I added 11x2.5" brakes to the back of my Challenger and paired them with the 11.75" rotors from the later cars. Using the stock prop valve everything is pretty well balanced.

I also use the 11x2.5" drums on my Duster, but I have Dr Diff's 13" Cobra rotor kit on the front. Again, stock prop valve, and again, pretty well balanced with the rears not locking up before the fronts.

The 11x3" drums should fit on your 8 3/4 just fine. You may need to experiment with the wheel cylinder size if you keep the stock front brakes and prop valve, there are a couple different rear wheel cylinder sizes that came on the 11" rear drums, both 7/8" and 15/16" were pretty common depending on the model.