[Synchro909]

What type of seat belts do want to install? IMO, the simplest ones are "lap" belts but they re pretty useless for the driver and front seat passenger apart from keeping them in the car. They are OK ish in the back.
Lap and sash belts are better in the front because they will prevent the passenger's face hitting the dash and the driver being impaled on the steering column but where to attach the top of the sash? From what I can work out, our regulations on this are more stringent than you guys have so you might be able to get away with attaching the top of the sash to the B pillar in a closed car. Doing so in a car not designed for that will weaken the pillar making the whole thing self defeating. BTW, it is NOT a good idea to attach a seat belt to wood, in fact it is banned here and any seat belt installation in a car built before seat belts became compulsory must be inspected and approved by a licensed engineer.
A piece of angle iron bolted between the chassis rails on each side is a good fixture and there is a pretty strong boxed section across the back of the body down at floor level. I have used that.
In my 1928 Tudor and 1929 Phaeton, I had made a special seat belt for the driver (me). It is "Y" shaped with the bottom of the Y anchored to the boxed section I mentioned and one of the top of the Y coming over each shoulder, attaching to a lap belt. It is like a racing harness and will stop me being impaled, should I run into anything. The down side of that arrangement is that it make things difficult for a passenger sitting behind he driver.
In the end, what you do is up to you but I think it is worth thinking about your legal liability if someone was wearing a seat belt you installed and was injured, They could argue that the belt contributed to their injuries and without any formal crash testing of your setup, you'd be at the mercy of a court. Good luck with that!!!