It will bolt in. No cutting or welding. I have never done one myself, but I have seen them put in Foxbodies. That is an awesome opportunity you have there. However, you might want to spend some money while you are at. For one, see if you can get some Cobra front brakes. Since the IRS setup will come with Cobra rear brakes, putting Cobra front brakes in will really help your braking system stay in balance. There are also a few trouble bushing and parts in that rear setup. I'm not sure exactly which ones they are, because I'm not too familiar with the setup. However, I'm sure you can talk to the Cobra guys to see which parts cause trouble. Since your friend has already been putting a lot of power through the rear end, it is a good opportunity to inspect it while it's out of the car and pre emptively replace those parts and or bushings that might cause you trouble down the road.
The IRS doesn't necessarily handle any better than the solid rear axle. However it allows you to make a car that handles very well without sacrificing as much ride comfort.
Kurt