Tout,
I am guessing that you mean taking a 1965 Mustang and putting a Cobra body on it and then using the Mustang data plate and identification. If that is what you are saying then you are back in that gray area and definately out of bounds in California. If that would work I have access to several older bodies and frames that a Cobra body could be put on but then I wold not be able to register it here in this state unless I did so as a 2003 Cobra replica. Can't do that because it won't meet all the new DOT regulations. Nothing short of a tank on DOT approved wheels will meet all the requirements.
Ron