If in fact your Ford Mustang needs a new catalytic converter you'll need to replace it with an OEM CAT designed to function with your vehicles engine and ECU. The only type of CAT you'll find in CA will be a California Air Resources Board (ARB) approved CAT.
49-State emission catalytic converters can not and are not sold in California (nor are they allowed to be shipped to CA). If you were out of state it would be a different story; you'd be able to replace the CAT with only one which was required for your vehicle (49-State Emission Certified).
The aftermarket OEM catalytic converter you purchase in California will be designed for use specifically for your 49-state emissions Ford Mustang. It will have an Executive Order (EO) number indicating it's been approved for use in California. You should not be replacing your vehicle's CAT with a 50-State emission CAT. You should use an OEM catalytic converter designed specifically for your vehicle which has an EO number stamp and has been approved for use on your 49-state emission certified Ford Mustang.