I have taken a lot of Firebirds apart. I have never seen a provision for a shoulder belt in the roof. My plans for my convertible always included a three point belt like the second gens used. That is another reason why I am adding a roll bar. It can be the shoulder strap mounting point. The first shoulder belt I came across was in my 1972 Catalina. The belt was removable from the lap belt, and could be stored in clips that were in the headliner above the window channel. I guess no one has a bare coupe to take a pic of that shoulder belt mounting area. I think seat belts became mandatory in 1965. People grudgingly used them at the time [some still don't] . I would be amazed that they actually provided a mounting point for a shoulder belt, unless it was a performance orientated car. That video says they did.