Re: why is rim width the issue?
You're not only trying to clear the outer 1/4 panel lip, but also the inner wheel well, as well as the lower mount shock bracket. Go look at a correctly back spaced wheel with a fat tire under it, it's a near miss with the lower bracket, and very close between the inner and outer fenders. You're limited to space because of the inner and outer, only way for exceeding the width from factory is mini or full tubing the rear and possibly re locating the shocks. Lots of dish is cool, but wheel rubbing sucks, and leave the jacked up rears and 4" of tire sticking out to the 80's. Even when you have a fat tire sticking out, the traction will still not be as good as a properly backspaced wheel that is smaller because the tire is sooooo far from the axle it can bend to the road!
The only Mustang I'd ever own is a Fender.
'68 333" Camaro
'14 5.0 Mustang