When installing seat belts you should consider the government design requirements when making the attachment points. The structural components of a pelvic restraint must each withstand a force of 2500 lbs. and the upper restraint (shoulder belt) a force of 1500 lbs. These are Canadian standards but US are probably the same. Obviously we can't design and test a compliant system, but at least build something that will give a reasonable level of safety over nothing at all.
I thought about cutting a slot in the door post of my Fordor for a steel bar also, but decided it was impractical and not strong enough. Instead I put a steel brace across the roof and attached the shoulder belts to angle iron brackets at each end. You can sort of see it in this picture. The force would pull down and forward. At best, I think my 3-point belts might reduce injuries in a low speed collision in a car never designed with passenger safety in mind. I hope I never have to test it.
I don't think my picture showed up - I posted one a few years ago, try a search for "seat belts" or "shoulder belts"
