It's a bolt in deal. There are many articles and threads about swapping an IRS into a fox car..
You'll need to modify your brake lines (unless the "donor" IRS still has the brake lines on it).
You might need to slightly relocate some fuel lines.. (just by bending them out of the way).
You might need to reinforce the upper control arm mounts on the car. And you'll need to fab up some rear mounting brackets, I think.
You'll also need a new cat back because the exhaust runs underneath the center of the IRS, rather than over the axles. A slightly modified Cat Back system from a 99-04 Cobra will work. Or you can just use turn-downs..
I just did this. It is not a bolt in deal. There is some minor fabing that has to be done. If you are going to pay a shop to do the work, it is not worth it. If you can handle it yourself or have a the help of a knowledgable tech, then go for it.
The upper control arms are not even used, so no reinforcement necessary. The only reason I did the swap was because I OT my car and I need rear discs. I happened to have a complete IRS that I got for cheap (plus some extras) plus some time on my hands, so I did it. The biggest PITA is the rear brake lines and the rear IRS subframe mounts.
I was lucky enough to be friends with the owner of Competition Speed and Sound, in Maryland. He was very accomadting with his shop and time to make this swap happen. We actually just got it on the ground yesterday and still need to sort out some soft brakes. I think they may just need to be bled.