Might I point out that if you are concerned about your homemade seat brackets supporting you and the seat with the seat belts attached to the seat, how are you so sure the seat brackets will keep the seat in its proper location in a crash? If the seat moves, belts attached to the floor pan are worthless.

How did you make your seat brackets?
If the seats are attached to the homemade brackets with bolts in the original location on the seat, and the angle brackets are bolted to the floors with at least 2 3/8" diameter grade 8 bolts per side, a 3" diameter washer or 1/8" flat steel plate on the bottom side of the floor pan will keep the seat from moving, provided the floor pan remains intact. Modern day seat mounting brackets are attached to the floors on reinforcing plates that run from door to trans tunnel. 2" x 2" x 1/8" angle is heaver gauge metal the the seat frame is made of, and is thicker material then the floor pan is, unless its at a reinforced pan are.

The seat belts have been added to the seat frames because it is safer. Belts attached to the seat keep you in the seat, and don't loosen up nearly as easily. Seat mounting brackets properly secured to a solid floor pan stay put in most accidents. Given a choice, if things fail, I would much prefer the added protection of me and the seat staying together rather then having a loose seat beating me up as well as me flying around inside the car.

As a side note, modern day (since the early 90s) seat belt retractors are much more sensitive to locking then the earlier versions were. Modern ones lock with a swinging weight on a lever. If it gets the least bit off center, it locks, until then its pretty easy.

I've been involved with a few rod builds that have been real life crash tested. Gene