Bracing for shoulder harness seat belts. I started an update on this a while back and noted that once I had located the correct attachment point for the shoulder harness (above your shoulder), I would be adding bracing since you can't expect the fiberglass to hold up in the event of an accident.
The basic idea is to make a steel plate that matches the curve of the fiberglass at the top of the package area behind the seats. The shoulder harness support will bolt though the fiberglass and then through the steel plate. Some conjecture that that would be sufficient in case of an accident. I had my doubts so I fabricated some supports out of angle iron and T iron.
Location of shoulder harness support
Unsupported curved steel plate test located in area behind the passenger side package area. It is bolted through the fiberglass to the shoulder harness support. Note that some of the side fiberglass had to be opened up to allow the plate to wrap around.
Finished supports: angle iron and T iron was cut, twisted and welded to provide front to back support for the curved plate. The T iron provides strong resistance to lateral movement. These are bolted to the chassis in the areas near the rear shock absorbers. They fit under or over the supports for the rear body. You may need to trim any spacers if your car has them so the supports can fit underneath. If you don't have spacers, the supports can be sized to sit directly on top of the rear body supports.
Brace mounted on rear body attachment points. The brace will actually be mounted after the body and chassis are secured together since you have to do a little bit of alignment when inserting the curved plate behind the package area.
If you choose to emulate my foolishness, I highly recommend having someone who can weld do your bracket work. I know of no one who has actually crash tested a design like this and I hope I never do so. So, no promises that this is the best way to approach the problem (actually, mounting on a roll bar is the best way if you want to go that route).