Jonathan - Theoretically, yes, you could put the Cobra IRS into a Fox-body Mustang. As much as the outside of the car has changed in the past 8 years or so, the underpinnings of the Mustang (even the brand new ones) are still based on the '79 Fairmont. Sweet, eh?
Anyway, the problem with trying to do that conversion is that it's not a straight swap, and a very limited number of people have done it. Any sort of conversion stuff would have to be custom fabricated. The Max Motorsports TA and PB is a straight bolt/weld-on affair. There are other problems, too, like I think the rear track is quite a bit wider - 3" maybe? - so the fenders would have to be flared out. That wider rear track would also make the car's tendency to understeer even worse (of course, the added grip of the torque arm will do the same thing to some extent, but it will be easier to tune around that). And, of course, the cost of a complete Cobra IRS is very high at this point, because there just aren't a whole lot of them around to scavenge from.
It would definitely be sweet, but you'd have to have some serious $$$ to get it all set up right.