The car originally would have come with seatbelts from the factory. However, most of the time these cars are restored the seatbelts are not installed back into them.
I can speak for the 71 Chevrolet models and the front seats came with seat belts, but not the back. I'm pretty sure it was an option. My old 71 Chevy came with lap belts and another set of belts (went across your shoulder) that folded over the doors. I was told by an officer after being pulled over that I technically did not have to wear my seatbelt in the car if I did not want to.