Actually, the Cobra rear calipers are the same as all the other cars, they're 38mm Vargas. The only difference in the Cobra brakes is that they use an 11.65" vented rotor instead of the other cars' 10" solid rotor. The vented rotor is thicker, because they need the vent space in the middle of the rotor, so the Cobra simply uses thinner pads in the caliper. That's why you have different part numbers. To put Cobra rotors on your GT or V6, you would need to replace the bracket, and of course the rotor and new (thinner) pads. You could reuse your calipers.