the law says you can only retain the original reg if a 'NEW' shell is used! ???
There are probably plenty of cars,not just classics that have been re shelled out there,if the only thing to ID them is a rivet on or spot weld on chassis plate its easy.
I dont mind admitting I did it with a rusty '75' dolly sprint and used a 'free' 25k '80' 1850hl to rebuild it with. I told the new owner though!!