I had a '61 Falcon, in Indiana. Considering that we have no inspections here, and cars in all kinds of crap shape, plus, no motorcycle helmet law for people over 18, surprisingly, I was required to install lap belts. I was not required to install shoulder belts, because it was not physically possible-- there was no where to attach them. I don’t know that if there had been, I would have been required to install them.
There was no seatbelt law in the state at the time for people over 16, so I don’t know why I was even required to install them. I guess the passenger side made sense, but I had to install one on the driver’s seat as well.
Never got into an accident with it, but I did have a few short stops, and they always held.
However, the Falcon was registered as a regular car.
When you register a car more than 25 years old in Indiana, you can register is as a regular car, pay the usual fees, and must have all the usual requirements, which include seatbelts (but not, apparently, doors). There may be a few things you need that I don’t know about because the Falcon wasn’t THAT old.
Anyway, if you register it as a vintage car, it’s cheaper, and you get a special plate, and the insurance rates are totally different (you can’t insure it for its replacement value as an antique, unless it’s registered as an antique); they go up, not down as the car gets older. You also are limited to putting only something like, 500 miles a year on it, if it’s registered as an antique, so the highway fund, or whatever it’s called isn’t part of the registration-- one of the reasons it’s cheaper. You also do not have to make “modifications” that make it “roadworthy,” or conform to safety laws, which I presume would mean putting in seatbelts.
At the time (1995-2005, more or less), I didn’t have to put a passenger-side mirror on it, but that may have changed, now that there are so few cars on the road that did not come with one.
FWIW, had I not been required to put in lap belts, I probably would have anyway.