An 8.8 is almost as strong as a 9". If you plan on going faster than 10's; then you get a Currie "bolt in" nine inch. If not; the 8.8 will do quite nicely with some minor mods, (i can go into the details later if you like).
An 8.8 from a V8 '86-93 Mustang runs about $150 from any Pick-a part; complete from drum to drum. Often when found; the diffs have had a gear change. Last weekend there was one with 4.11's in the Pick-A-Part here in San Berdo......
Another option is to bolt in an 8.8 out of an '87-88 Thunder chicken turbo coupe..... The auto cars have a 3.73 LSD & the stick cars have the 3.55 LSD....... Both have rear discs as well.... The T-bird can also donate the 2.3 turbo setup.... If interested in the turbo coupe rear diff swap; go here:
http://mjbobbitt.home.comcast.net/~mjbobbitt/