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 ??

If you have a choice I would go with less backspacing. It saves some weight if nothing else.

Narrower rear end is lighter and stronger. On the other hand a centered wheel is probably the strongest and least flexible.

The deeper the offset the easier they are to remove, usually work:: up:

I'm wondering if that much back space is actually available.

My race car had a set of Monoque that were 15 inches wide with 3.0 offset from the back side
I have two other sets of drag race wheels, a set of Centerline 15 inch wide with 5.5 offset and a set of Welds 13.5 with 4.0 offset.
I think Weld offers offsets in half inch increments and Centerline use to only offer 3 1/8 and 5 1/4 inch offset regardless of the wheel widths, I think: work:

7 1/2 inches? on a 10" rim?

1/4" of tire clearance to the wheel tub? Seems like that would rub when cornering?

I'd find the rims you like, then that makes the rear end choice for you.
You may change rims over the life of the car, so pick something common/standard
Only want to buy the rear end once........

Not uncommon on later vehicles - Mustangs in particular.

The original poster was talking about 12" wheels.

Otherwise you might have to order custom offset wheels from somewhere like Holeshot. Not a bad thing, but expensive.