I think you could do it, if I remember correctly the cobra IRS uses the same chassis mounting points. If I also rememeber correctly, the geometry for the IRS isn't that great, not sure why or how it would impact the car in autox though.
You def want some subframe connectors, I've seen some funny pics of old floppy fox bodies on r-comps :D
Parking Lot Hero
2008 WRX daily driver
2003 ZX3 23D Autox Car 42 FSP
"Hella Functional - Raced, Not Hard Parked"