Ok ... '69 Road Runner. Mini tubbed 2" (not all the way to the frame rail). Springs moved inboard.

With the stock width 8-3/4 I could fit a 10" wide wheel with a 5-1/2" backspace into the well with about 1/4" or so clearance to the inner tub. The tire in use is a M/T with a 13.1" section

With the wider tubs I plan to use a 12" wheel with a tire with a 15" section.
Its now time to order a new rear end and I have a few options ...

1) Order a stock width rear end and use a wheel with a 7-1/2" backspace
2) Order a 1" per side narrower rear end and use a wheel with 6-1/2" backspace
3) Order a 2" per side narrower rear end and use a wheel with 5-1/2" backspace

So there's my dilemma ...what to order. Thoughts ??