@Hoytster
I think it's time for you to join the discussion with your solid mount IRS. Dave is in the searching for big tires just like you
https://www.stangnet.com/mustang-fo...d-weld-spatter-in-my-ears.900417/post-9098471
So a hard tail mount is the main limiting factor for a wide wheel and tire?
I really haven’t studied this thorough enough. Today I’ll start looking at it deeper.
Then there’s still the issue of finding one, though I think that just takes patience as well.
I'm a little late to this conversation and am probably going to re-iterate some of what's already been said.....
With everything I've seen so far, the max tire width I've ever seen anybody run on an IRS equipped Fox body (with no fender flares, rolled inner lip, hard tail mount, some sledge hammer work) is a 295/45/17". I forget what wheels he was running, but I believe some spacers were used to get the correct offset. @stanglex2002 is correct, you really can't mini-tub a car with an IRS since the IRS cradle is what gets in the way. You're only other option is to go the fender flare route.
If I were building what you are building, the biggest reason I would switch to an IRS would be ride comfort. The difference in unspring weight between an SRA and an IRS is huge, and contributes a lot to how the IRS feels. The IRS is also a little more predictable when pushing the car hard through corners, you don't get the solid axle "side-step" when hitting bumps or going over curves. I went IRS because I will be pushing my car on the track, and I like to use as much of the track as I can, which involves lots of curbing. I am also looking to increase my overall track width and will be running Maier Racing fender flares, so the IRS was a perfect fit for me. I also live in PA, and we don't know how to fix potholes around here.
On a smooth road or race track, the difference between an IRS and a TA/panhard rear SRA (or watts link) is negligible. All other things equal, the IRS will be just slightly quicker the SRA. Start making quick corner transitions, running over curbing, or start hitting bumps, and the IRS will start shining. Start doing hard launches at a drag strip on stickys, and the SRA will start shining.
Almost all of the shortcomings of the stock IRS can be overcome with the right parts. Get rid of the soft rubber mounting bushings, strengthen the case with a good cover/brace, replace all the control arm bushings with delrin, and you will have a solid setup with no wheel hop. But, from my experience with a TA/watts SRA, you will not have the same amount of forward traction with an IRS as you will have with that setup.
I saw you looked at the s550 IRS swap into a Fox. I probably could have saved you some research on that one. I looked heavily into that and the amount of fabrication work that would be required to make it was counterproductive. Not to mention the s550 IRS was significantly wider than the cobra IRS and just would not work. I know a guy toward the Philly area that is doing that swap on a 70's Mach. He is cutting the entire floor pan out and grafting an s550 floor pan in so he has all the mounting points for the IRS.
Here is the hard tail mount I did on my IRS cradle. If you happen to ever decide to go the IRS route, I have dimensions and progress pictures on how I did this and can send them to you. You wouldn't have to go as overkill as I did, but could easily modify my design to meet your needs and make the fabrication a little simpler.
I'm not trying to sway you either way, both setups have their merits. Just need to make the decision as to what fits your needs (and budget) better.