Shoulder belts came along with the latches on hinged seat backs, 1968.
Sure the belt across your chest hurts in a collision. It would hurt a lot worse if that seat back goes forward with you. Only a crash test dummy knows for sure. LOL
Shoulder belts could be had in convertibles 68 -70 on A, B, E and some C bodies. The shoulder belt was usually mounted at the outer lap belt attachment and that the belt was usually about 52 inches long.
