A suggestion... I have a 78' Trans Am in addition to my 70' ss. I too have wondered about this as my shoulder belts are missing.
From what I can tell, my T/A belts would do the job. It basically is a one piece unit that has a retractor on the roof and the other end of the belt bolts to the floor on the outside of the seat. When extended, the latch pulls the belt over the <body> in 2 pieces and buckles right up.
Firebirds of this vintage are a dime a dozen in salvage yards. As long as the overhead retractor could be bolted to the roof, I think it just has 1 bolt, it should work.