If you're planning on adding an additional 3 inches to your existing 4" suspension lift, then you MAY run into some problems with steering angles (i.e. pitman arm to drag link, steering dampener to drag link, link pin height) and could possibly have to weld in plates to raise the affected parts. And I'm pretty sure that you might have to put in some shackle extensions (may put undo stress on the springs, and make your ride feel like garbage). I have installed lift kits and had no problems, and then installed ones that have become a total headache (I've wound up having to install longer steering shafts on a couple of Blazers, welding plates in on CJ's/Wranglers, installing longer brake hoses, adding in extensions on body wiring, excetera). It's pretty much a case by case basis. But as long as you'd be willing to modify your rig a bit (if needed), you'll be able to tuck some big meat in under the fenders.
As for the body lift itself, it's fairly easy to install, you just unbolt the 4-8 bolts that hold the body to the frame, lift each side of the Jeep body up, and slide out the factory body mount bushings, and slide in the replacement bushings that you get in the new kit. Install the new body mount bolts that you'll get in the kit with the bushings and you're done.
ASE certified mechanic with 15+ years in the business, specializing in suspension, and a die hard 4x4 guy