Welcome to the Team Jon!
Shoulder belts were only offered as a dealer installed option in 1972 and didn't become required equipment until 1973. In fact in 1967 seat belts were being used as standard equipment for the first time. Rear seat lap belts were not offered as an option until 1968, so there are no mounting point at all in the back for seat belts of any kind from the factory. There will be no shoulder belt provision for your year car from the factory; so you will need to go to the aftermarket for both belts and the mounting hardware (inertia reels and anchor eyes with floor board reinforcements; to keep the anchor from pulling through the sheet metal it is attached to upon impact.