This backspacing doesn't sound right!!!

8 1/4 read end! I'm just going to get the same demminsons as the wheels that came with the rear end since I know for a fact they work with those tires.

Well that is the safe thing to do. I have a 8 1/4 in my 69 with Ralley 14 x 6 wheels. I know I have room in there for a wider wheel but not sure how much.