I did an irs swap to my bullitt this past summer, and the riding and handling improvements are immense and the project was for the most part very easy. I used an 03 cobra rear end, but any 99-04 irs will work. The primary differences are the the 03-04 have stronger half shafts, beefier diff mount, and 3.55 gears.
I definitely recommend upgrading all the bushings/diff mount/toe bars while the cradle is on the ground. I went with all prothane bushings because it is not a dedicated track car either, but some people swear by the aluminum bushings.
I used the rear 03 cobra coupe springs and the front bullitt springs because that gives you the same spring rates as the 03 terminators and koni SA shocks/struts. I also used the factory Cobra rear bar with the factory bullitt front bar. the car rides much better than it did before, corners flatter and does not get upset easily, and feels mostly neutral at its limits.
The trickiest part of the install was the rear springs. You have to mount the front of the cradle, set the springs in the control arm, and then jack up the rear of the cradle with the springs. If you have the car on jacks stands on the frame rail, you will have to put a jack under the K member, or you will lift the car of the stands.
If you have all the parts and two people with relatively good mechanical skills, you could do this in a weekend. It took my dad and I a few weeks because we had the transmission out for a rebuild and had issues with some parts taking much longer to ship than expected.
