Has anyone upgraded to the 11 inch mustang ll brakes that are available everywhere? Of course then you'd need 15 inch 5 lug with the correct off set but stopping power would have to be much better especially on V8 cars.
I'm sure my II could use some more stopping power though it's not as bad as my '69 Mercury with 4 wheel drums. That can induce some panic so that might get an upgrade before the II does.
I've noticed some of the kits (Speedway has one for not very expensive) that use the GM calipers (those weren't all that awesome to begin with but better than the II's) have rotors with 5x4.75 bolt pattern. That'll make it a bear to match wheels to the rear unless you plan to redrill those as well, and I don't know if those will do a 5x4.75 without getting too close to the edges (Mustangs have been 5x4.5 / 5x114.3 for forever now).