The thing about modern seatbelts in an old truck is that the seat frame attachment will fail. Cushions may not be fastened. This can cause major injury. The seat bolts/floor threads were not designed for todays possible "inertias". Many trucks do not have a lock on the seatback either. Probably better than nothing, but will be a hybrid system.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn