Absolutely agree and is why I thought the bolt through the roof system inadequate. A lot of structural engineering goes on to safely stop a body nearly instantly in a collision, holding it back and not forcing it down. Properly installed and worn, the seat/shoulder belts work. I always thought the power rail set up Ford had worked well, took a bit to get used to though for me.
When working I had access to a lot of engineering data (nearly anything I would ask for) and the issue of spinal compression was a major concern. Granted the magnitude of the liability is tremendous for a manufacturer and would not extend to the modification of an old car, but the info was available.
The simple seat and shoulder belts do work.
For the record I have not added shoulder belts to either of my Thunderbirds.