Rear seat belts were optional in 1965, and holes were not drilled for them unless they were purchased.
If no seat belts were installed, you may have to weld a brace to the underside of the floorpan (the sheet metal that makes up the floors is not strong enough to hold a belt in place) and drill your own holes. Belts attach to the floor under the rear seat.
submitted by a65ragtop