Backspacking question - I need help PLEASE

Let me get this right... if you had the rear end narrowed 1/2", that means you had a 1/4" cut off each end... then got custom axles...... so you could get 8 1/2" wide wheels to fit.......and you're STILL having fitment issues........... REALLY?!?!

That has got to be the stupidest thing I've ever heard.

You know there are plenty of 9" wheels available that will fit a stock Mustang.... I know, I had some on one of my Capris.

And if you get a wheel with a 3 3/8" backspacing instead of a 5" backspacing, it will move them out 1 5/8" on each side.

The backspace is defined as the distance from the rear edge of the wheel to the bolt pad, so the 5" backspace wheels you have are better than the ones with the 3" backspace. If you really want that to work you basically have two choices: cut your rear axle again or order wheels with something like 6" backspace. But before you do any spending, measure very carefully to see what will fit. My method is to remove the wheel and tire and hold a long straight edge ( a carpenter's framing square works well) against the bolt surface of the brake drum. Then, with a tape measure, find the distance to both the inside of the wheel well and the outside of the fender lip. Then subtract an inch or so from each measurement to be safe and to allow for lateral movement and you'll have both your width and your backspace. Hope this helps, Jim

It would be risky of me to make a suggestion and even riskier of you to take my advice since I can't see and measure your car myself. But here's a thought: right now you have 8.5" wheels with 5" backspace, right? So that means you have 3.5" of front space. If you go with the 17x9's on your list with 6.8" backspace you'll now have 2.2" of front space, or a bit more than 1 1/4" less than you have now. Will 1 1/4" make the difference you want? If so, that would be my choice, but you need to be very sure it won't hit on the backside with 6.8" of backspace and your narrowed axle. Don't just guess, yank the wheels off and measure because it's going to be and expensive mistake if you're wrong.

Sounds like your custom setup has complicated things.
If i were you, i'd find someone with some cobra wheels, test fit them, and use the wheel specs and measurements as a baseline.
Since your rear has been modified, i'd say any other way will just be a guess.