I had the joy of replacing the body mounts in the jeep this week. Broke off 4 bolts in the process. 2 of them had enough left to weld a nut to what was left and they came out willingly. The 2 rear most bolts snapped right at the bottom of the nut plate. Of all the ones to break, these are the best ones since they don't require you to hack a hole in the tub floor.
Only took one picture, but basically you just cut a mousehole where the nut plate is - plasma cutter works best, but a cutoff wheel could be used to make 3 cuts. Peel the mousehole down with a screwdriver. Pound the nut plate loose by sticking a big screwdriver up through the body mount and whack it with a hammer - once was enough. Once you have the nutplate out, then grind the remains of the body mount bolt off each side of the nutplate. Drill a 5/8" hole through where the old bolt was, and weld a new nut to the plate. You can tack weld the plate back down where it was and then fold the mousehole closed. Havn't decided wether to bother welding the mousehole shut.
Sounds like a long process, but it took about 15 minutes per side.
Mousehole:
