Earl, to quote Master Power Brakes directly from their website.........."When using disc brakes, you cannot have too much assist in the system"
I have a dual diaphragm 7" booster from Master Power Brakes and because my camshaft doesn't make the required vacuum, I have installed a vacuum pump and I have 12" rotors all around with 4 piston calipers from Stainless Steel Brake Corp. The entire brake package has performed flawlessly. Save for the Vacuum pump, shortly after install the vacuum pump shaft sheared leaving me no vacuum therefore it took quite a lot of foot on the brake pedal to slow down.
Pretty much what you speak of. You need a booster I have the same car with the same Mustang II upper and lowers and I didn't have to grind on either for these brakes to fit. They are incredible and I highly recommend them. You can steer lock to lock without an issue, nothing touches. Of course you'll need 17" wheels to clear the disc's
Your 9in discs all around are more than adequate for a car as light as a Cobra, but install a booster. It is a night and day difference
Visit
http://www.mpbrakes.com/techtalk/boo...ur-classic-car
