This is unsolicited advice so feel free to ignore me . . . my Wife does it all the time.
For all the headaches involved with stuff the IRS into a Fox combined with your desire to stay 4 lug, I would highly recommend considering spending the time and the money on installing the Full Torque Arm, Panhard bar, and rear lower control arms from Maximum Motorsports instead of the IRS.
I have an 03/04 Cobra IRS installed in my 79 PC and I have another 2001 IRS sitting under a bench trying to decide what to do with it. My .02 is that unless you just want the stock IRS with none of the OEM flaws corrected, then fine. Problem is that at this point the newest version is 16 years old, so . . . You are most likely going to need to install all the bushings, the cross-axis joints, tie rods, etc. for a good all around unit. By the time you do all that, you will have spent just as much for those parts as the MM Torque arm setup and required parts. In my experience I will just save the headaches and do the full MM setup and not have the tire fitment issues and backspacing issues, etc. NOT to mention all the brake issues and machine work you have ahead of you. Oh don't forget you have to have IRS specific tail pipes too.
The IRS is nice and it's a novelty item, but a well tuned Torque Arm will do just as well and be fewer headaches IMHO. Again just my .02 and I don't know how deep you are into the IRS at this point in time, so if you are dead set on the IRS, then just ignore me and my ramblings.
Best of luck either way!
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