[Three Peaks]

No point in reinventing the wheel, so to speak. FFR already has a fully developed IRS cage with suspension pieces and all that just welds in place and you supply the differential, uprights with hubs, and CV axles. You can pick up a used Tbird Supercoupe (89-93) IRS for around $200-300 and just clean it up and install it per instructions. If I remember right, FFR also supplies the shorter axle shafts for the CV's and the shocks for the IRS set up, but you should check on that.
The CWI really isn't designed with the FFR chassis in mind and would take some serious fabricating skills (read expensive) to be fitted to the FFR.
The newer SVT Cobra IRS can be used, but some of the parts aren't compatible- such as the uprights and the 31 spline CV's in the newer Cobra differentials. Up to 2002 or 2003 (can't remember), the Cobra IRS had 28 spline CV's , same as the earlier Tbird CV's. So, you'd be out the cost of the SVT IRS, the proper uprights, CV's and othe miscellaneous parts.
You could probably get a lot more information on your FFR at the FFR forum ffcobra.com

Bob