Griggs makes several good points. The caveat being that he is really addressing most of this from a competition angle and not a street car angle. This is especially true in regards to a Fox with the IRS since the IRS was an add on by Ford to a chassis that was originally designed in the late 70's and was never intended to have an IRS much less be a real road course handling car by any means. What Griggs, Maximum Motorsports, Agent 47, Kenny Brown, and others have been able to do with the Fox chassis over the years is really pretty amazing when consider the humble beginnings of the design.
With all that said, I think the big question for you is what do you plan to do with the Mustang. Is this a street car or a track car or a hybrid of both? If this is a street car, then full on race tuning is neither needed or desirable in most cases. If this is a hybrid, then you will want to consider many upgrades to the IRS, if this is a full on race car, then you might want to reconsider the "Perfect Paint" if going Wheel to Wheel.
Maximum Motorsports and Full Tilt Boogie Racing both offer several upgrades to the IRS that will improve the whole rear suspension even for the street car. Not necessarily cheap options, but definitely make improvements over the compromised OEM design. Everyone here knows I am a big proponent of Maximum Motorsports and their products, but I do have to admit that FTBR has done a ton of hands on work with the IRS and makes some very nice products. I have a few of them on my 79 PC and as the budget allows there will be more.
I do have to agree with the statements above that doing a 1.5" flare kit to your Mustang may be the better option for what you want to accomplish with this Mustang. I would recommend the Maier Racing flares as IMHO the 1.5" kit is subtle but does give that added width that allows much wider wheels and tires at all four corners. YES, the kit will require some extensive body work to be perfect, but you have shown that is an area you are highly skilled in and shouldn't be an issue. I currently have their 1.5" fenders on my 85 Road Racer GT and I love the look, most people don't realize their are wider than stock and they look great. I still need to do the rear quarters, but not being home in CO and budget have not allowed that to happen just yet. I will say that with my experience with my PC and the IRS, adding the rear quarter flares should allow you to easily run a 10-10.5" rear wheel and possibly even an 11-11.5" if you go to the hard mount on the rear cradle with no clearance issues. That will be plenty of wheel for 315 tires and give you a subtle wide body that just looks killer.
That would allow you to install the IRS as is without any need to modify the control arms, shock mounts or even Mini Tub the Mustang unless you just want the additional work.
The flares will work with your stock front and rear bumper covers and all the stock side moldings, etc can still be attached to look like an original Mustang if you choose. I highly recommend checking into a Maier Racing's Website.
https://www.maierracing.com/product-...79-93-mustang/
Here is a picture of the Maier Racing 1.5 fenders I have.
Attachment 120422
Here they are on the car with stock rear quarters at this time.
Attachment 120423
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