Snowcone,
I had seatbelt installed in my mainline in the weeks after I bought it. I went to a place that specialized in baby seat installation that also did seatbelts. I got three inertia reel belts across the front
I had them installed for three reasons. safety; obviously for my then wife and stepdaughter, time; I took the car on a holiday trip just two weeks after buying it, expertise in fitment; to get the right points and ergonomics right.
I would guess that along with brake improvements and chassis changes that safety modifications like seat belts would need to be compliant with standards. Check with the RTA or inspector. Personally I can't see the point of trying to save on safety related items.
I think seat belts were part of the Lifeguard pack introduced in 56. So look at a 56 or later car for comparison.
cheers
Warren
Grizzly (Aussie Mainline)