Legal question about Antique vehicles and the seat belt laws

Any law enforcement men or women and anyone else who know the answer. My question is for instance my 1966 van I have licensed with antique plates in the state of FL. I have put lap belts in front. No seats in the back , In the FL rest stops along the highway on their bulletin boards states people over the age of 8 or 12 or something (age not important ) are not required to wear seatbelts in the back seat in FL. Is it LEGAL in other states for an ADULT to ride in BACK if the van doesn't have seatbellts ?