It's been a while since I took the factory bumper hitch off my rusty 87, but as I recall I had to torch off the torx bolts. The rest of the fasteners weren't too bad despite the rust, and if you're lucky you might be able to recycle the nut strips inside the frame, which are the same as those used on the class III hitch.
Handy hint for recycled nut strips if the threads are tired, rusty or stripped. The metric fasteners used for the hitch are just a little smaller, and the same threads per inch, as 3/4" US coarse thread. It is very easy to retap the holes for US 3/4 x 10.