I originally built my car with stock single piston calipers in the front, and stock T-bird discs/calipers in the back. Braking was excellent, and easily controllable. On the street I never wished for more brake, only more traction. With stock Mustang brakes, you should be able to easily lock the tires at any speed less than about 45mph or so.
On the track..........
The point is, if you have stock brakes and they don't work well, there's a problem with the set up, not the parts choice. Rather than simply throwing parts at it, you should try and figure out what the problem is.
I'v done pretty extensive reading over the years, because this is a common problem. Do a search on any cobra board and you'll find hours of reading. The two main culprits are 1. wrong pedal ratio, and 2. wrong MC bore size. Check those two things out first, and you'll be time and money ahead.