Iirc, if the car was manufactured without seatbelts (read before they were mandated), then you are not required to wear them in such a car. I wouldn't imagine you would be pulled over in a classic car that meets criteria if the officer knows anything about cars, and if you are pulled over for it, I'm sure after some checking into it you would drive away without a ticket (for a seatbelt anyways).
Back on topic though, I thought some time ago that they had passed a law that you could be pulled over for just a seat belt offense, that's why I never fought it when I got pulled over a few years ago when my wife wasn't wearing her seatbelt (front seat passenger), and the officer told us that's the whole reason we were pulled over. I just paid the ticket anyways because she was in the wrong.