Be honest, it would look fake, just as the disappearing car in one of the new James Bond films looked fake. . Stunts, human or mechanical now look awful and nothing special. This of course is a personal view but I would rather see real old cars pushed to the limit for realism than obvious fakes, that is to say were it a gangster film, an old Nash or Lincoln being used in a cops n robbers chase with the inevitable crash, You couldn't really see Al Capone or his henchmen, being driven around in an armour plated Model A could youThe wonders of modern computer technology might make it possible, that no real cars need to be "killed off" any more. If they blow up a vintage car, they can do it in photoshop.
When they crashed the Ford in Chinatown, this was a rare car even at that time, but with a multi million pound budget, would a film company really worry about damaging or scrapping an old car if it was essential to the plot? Such as the cars in the mountain scene fro the original Italian Job, was that a real Lamborghini Miura with its engine removed they chucked over the cliff? What about the Aston martin arguing with the JCB? Or would it have been better using non prestige cars for this scene with rich Italian gangsters.