My understanding is the 94-04 Cobra/Bullitt/Mach 1 brakes are identical in function. They all require the same 13" rotors and all will bolt up to our spindles. I don't know about the piston size, never heard that before but that doesn't mean it isn't true.
Of course, the Cobra calipers have "COBRA" etched into the caliper, the Bullitts are painted red and have the running horse logo, and the Mach 1 calipers are plain unpainted. You can always paint the Mach 1 calipers red or any other color.