I'm not sure if anyone has done this. First some history, a few years back a friend with a '54 Corvette had a driver run a stop sign right in front of him. He t-boned him and ended up hitting the steering wheel very hard. The car was repaired but it took a long time for him to heal. When the car was repaired he had shoulder belts installed. That got me thinking, we put a lot of miles on the Firebird (4-5,000/year) going to shows and just enjoying the car. It seemed to me that shoulder belts would be a good thing.

A friend had a 2000 Grand Am that was about to meet the crusher. He said I could take anything I wanted so I grabbed front end parts for my GA and also the belt retractors, seat back positioners and other necessary pieces. I was able to fasten the retractors directly under the ashtray requiring only that some material be removed below the bolt hole on the retractor, the unit is rotated 180 from its application in the GA but operates the same either way. The top of the retractor was fastened to the support brace.
The next step was to get the belt out into the open. I decided to bring it up between the arm rest and the side cover , where the window crank is. To keep the belt from pinching I modified the guide that was in the GA to close its gap down to where the belt just slipped through. This is held in place by friction.
The belt positioner from the GA attached to the headrest with two holes. I had to drill a new single "slot" to attach to the '69 headrest. The plastic piece also had to be re-shaped to raise the belt to a more comfortable position. This was done with a heat gun. The angle was changed about 30 degrees. I didn't take any pics of the " before". The end of the belt attaches to the same position of the old lap belt.
For the receiver end of the belt I used some from an '08 Grand Prix because they are more flexible. They are attached to the lower back portion of the seat. The receivers have a pre-tensioner which is disabled with the wiring being disconnected.
All in all the installation looks very good without having to cut any panels, things could be put back to stock with no visible signs of the mods. The hole in the carpet where the old belt fastened to the tunnel is visible but the carpet is fading and should be replaced anyway.
I realize that without crash testing the true value of the belts can't told but I feel that in a crash they will add some safety. The weak spot is probably the belt positioner on the seat back but again it has to be better than the old lap belt only.