The backing plates on the seat mounts and the factory seat belts, are rather small and attached to the floor sheet metal. Collectively, they provide the "tear-out" resistance needed, in the event of an accident. A backing plate, similar in size & shape, as the factory seat belt mounting backing plate, is sufficient for the B-pillar on the 3-point belts. The 3-point shoulder/lap belts were an option and the Accessory Installation Manual, indicates the backing plate at the B-pillar.
