I've got the 13" Mach 1 front brake setup on my 2003 GT...of course, it consists of the rotors and the proper calipers. Stock GT calipers will not work.
Only "extra" thing I got were the stainless steel braided lines that run between the hard line and the rotor. No extra brackets necessary, at least not for the front.
Now if you wanted to run the Mach 1/Cobra rear brakes, you'd need the bracket for the calipers.
Like everyone has said, you just need the cobra calipers, 13" rotors, stainless steel lines..... and that is it. A really easy swap for the fronts. Search it and you will see that you can do the swap for ~$600 yourself. Look up buyfordracing.com for the calipers (already come with pads), rotors and lines from americanmuscle.com, etc.