I think I solved the problem. It cost a bit of dough though. My plan was to install the cobra bullit rear 11.65" rear disc brakes. I needed the cobra mounting brackets when I ran across this...
North Racecars Brake Solutions
I bought the kit with axles brackets and lines to keep the stock fox width and go to a 5 lug cobra rear disc. The fellow there claimed that in most cases you don't need the antimoan brackets because the axle is pulled in shorter and there is less flex in the shaft.
I mocked up the brackets, axles, calipers and wheels and slid it under the car. The fit is great if not perfect for my boss 338 wheels.
So you could buy an 8.8 from a fox to SN95, buy this kit and the calipers and be good to go for a 67-?? mustang. Many fox guys buy the calipers rebuilt from autozone or oreillys and save even more than I did.
The trick is finding a good used rear with the gears you want. I want to point out it looked like my axles were used with some machine work on the hub end and new studs. I could be wrong about that but that is what it looked like to me.