Using mustang rotors for 4.5" bc

I have a 64 barracuda that has front disc's when i bought it a few months ago. It is sbp 5x4", does that mean it is the kelsey hayes system? How can i confirm this before i start buying mustang rotors and pulling stuff apart.

Yes, you should have the Kelsey-Hayes system. Original calipers will often have a KH cast into them.