65 barracuda need help with rim backspacing

OK, so I have an E-body housing for my 65 barracuda and I want to narrow it to the stock A-body length so I can buy stock A-body axles instead of getting custom axles at a greater cost. I took some measurement of the rear tubs and the axle to see what kind of backspacing I would have to get on the rims and how much rubber I can stick under the car. The rear spring are getting moved in to the frame rail and I have a set of SS springs.

The stock 8 3/4 drum to drum measurement is: 57 1/8"
The 7 1/4 drum to drum measurement is the same at 57 1/8", so I took the measurements with the 7 1/4 in the car.
The tubs are 12" wide, from the axle flange to the outer fender is 5 1/4" and to the inside wheel tub it is 6 3/4.

I think that the maximum tire sectional width should be about 10.5" which means that I would be able to use either an 8" or 9" rim. As for the backspacing it would have to be 4" plus 3/4" (on an 8" rim) because the axle flange is that much closer to the outer fender. And it should be 4.5" plus 3/4" on a 9" rim.
I also want to run the tallest tire I can because I have 4.30 gears in the pumpkin that is going into the 8 3/4.
If this makes any sense whatsoever to you guys can you let me know if my calculations are correct and what kind of backspacing do some of you run on your 64-66 barracudas. Thanks, sorry for the long explanation.