Seat belt laws?

My 65 Dart didn't have a shoulder belt attachment point on the roof, and I saw no easy way to add one, so I looked for convertible seats. I got Cobra seats cheap, with a shoulder belt loop at the top of the seat. I haven't found belts yet, but many will work. More vehicles have shoulder belts integrated into the seat lately, even sedans, but some look a bit odd with the shoulder belt assembly bulging on one side.