Mustang GT rear rotors are smaller in diameter, and solid, not vented. So that would be a brake downgrade, not upgrade. Calipers are pretty much the same units as the Cobras, so the same issues with parking brake cable attachment (and smaller pad size) would apply.
There are no off - the - shelf 5 on 4.5" pattern brake parts that are the same size as factory SC rear brake parts, as far as I'm aware of.
cheers
Ed N.