Call me if you want to discuss this further.
Just use wheels designed for an S197 car when installing an IRS. These wheels have plenty of back spacing. Use the same wheel in the front with a spacer. Problem solved.
Wheels become less of an issue if you broaden your horizons beyond commonly used Mustang wheels, specificly; foxbody wheels. Don't look at LMR or American Muscle under the 79-93 section. S197 wheels are an option but they all tend to be 18 inches or larger.
I have 17x7.5 crown vic steelies on my car. They have a +54mm offset.
I ran a half inch spacer in the front so the would clear the coil overs. They fit great, and looked cool. But were narrow and I wanted more rubber under the car. I couldn't bring myself to drop the coin on a set of custom CCW's, so I was planning on buying another set of steelies and taking the two sets and making one set of 17x9's, and moving the center to get the required offset front and rear. I know people with the tools and skills to handle that, and I feel with their guidance I could handle the project, but not having the tools at home doing it would mean interjecting myself into a buddy's life. I decided I didn't want to do that unless I couldn't find an off the shelf option.
I had a foot injury that kept me camped out on the couch, so I began reading up on every IRS swap thread I could find and figuring out what was the biggest wheel and tire combo to fit and what the wheel specs were. I saw in one thread Jack mentioning that S197 wheels were a solid option. I saw AM sells Saleen SPeedsters in s197 fitment, but in 18's.
I want 17's. So I dissected what those wheels were, what the wheels I had were, ad what I wanted. I read up on terminology (backspacing vs offset.) I figured out I wanted a 17x9 with a lot of offset. Most foxbody wheels are +25ish mm offset. That's not enough with the IRS to put the tire IN the wheelwell. S197 wheels are +45mm, my wheels were +54 and well tucked in; so I needed +45 or MORE offset. Too much offset and I can just use a spacer to put the wheel where I want it. I began looking for 17x9 or 17x8.5, 5 on 4 1/2 inch (114.3mm) bolt pattern and +45mm or more offset.
There are a lot of options out there. Honda S2000 takes the above fitment, just narrower. Enkei, BBS, TSW and other make wheels that fit. You gotta pick what you like, what you can afford, and what fits. Unfortunately you can't just order up $175 Chinese 93 Cobra replicas from LMR and put them on an IRS car. You gotta think outside the box.
I picked 17x9, 5 on 114.3, +45mm offset Enkei RPF1's wrapped in 275/40/17 Nitto NT05's. I'm fully expecting to put spacers on the front, and maybe on the back. I'm fine with that, I will buy or make spacers as needed. I have a BFH for the rear inners as needed, I also have a fender roller to roll the fender edges. I'm expecting to have to add some bracing to the front fenders to move them where they will be out of the way of the tires.
New front springs and rear shocks and coil overs will be here next week, and tires will be mounted so I can start putting everything together.
TL;DR: IRS wheel options are out there, just look for an offset of +45 or more and use a spacer as needed if you have too much offset.