Hi
In a previous car, Porsche, I fitted 5 point harnesses with the crotch strap coming up through the bucket seats base.
I also needed to weld new harness location points onto the floor and sills.
There is a 964 911 part code, cars built between 1989 and 1994, that is a small plate with the harness thread insert fitted into it. The plate is around 30mm by 45mm and can welded anywhere. It is also quite cheap, for Porsche, circa £5 each.
I welded 3 per side, driver and passenger. One in the floor, one on the tunnel and one on the inner sill.
I could mount them anywhere I wanted so keeping the original seat belts in place as some MOT testers get sniffy if you turn up with harnesses in a road car. Something about being able to click and unclip a seat belt with one hard.
I do not know if your aware but seat belt mounting bolt threads are universally standard irrespective of the manufacture, original or after market. The Porsche part supports this thread.
Just my experience and could be of use to someone.