At the risk of taking this thread totally off topic, my '84 Allegro only has 4 belts in it. 2 up front (obviously) and belts bolted to the floor near 2 unattached swivel chairs. Seems silly to me that you'd strap your self to the floor while sitting on something that isn't firmly attached to said floor. With enough inertia, the swivel chair will allow the seat belt to cut you in half. I guess the idea is not to fly out of the thing in the event of a collision.
All that being said, my family is 5 people big. Thinking about installing belts on the couch, too. The couch is bolted down, so it would be at least as good as the Tiffen-installed belts.