On the shoulder belt coming undone from the buckle I can shed some light.
There was a factory recall in 1971 that involved several Ford models. I had a new 71 Maverick at the time. When I took the car in they simply put O-rings around the headed stud to prevent it coming off as easy. They had an inside diameter that fit the small diameter of the stud on the shoulder belt and you just stretched them to put on.
Hope this solves your issue it worked back then.
Those are some neat looking solutions to new belts and are much safer I am sure.
I think Ford first put belts in 1956 but not many sold. The first car I had with them was a 59 Ford. They did not buckle the belt slipped in between the buckle and when you pulled on it they would tighten. I would think that they would pinch the belt into on a hard crash. No real standards early on.
One of the few Inch bolts on a BMW use to be the seat belt anchors. I think 7/16-24 threads. They might have changed now. I was told all the testing had been done with that size and they just did not change.
David
I went out to the mighty web and found something on the recall for the shoulder harness. They said it did not affect the Maverick but I know they recalled my 71.
Oren09/Wikipedia
Slide 5 of 12
7. Ford (1972)
Vehicles affected: 4.1 million
Issue: Connecting part in seat belt breaking from wear
Components affected: seat belt webbing
In 1972, Ford Motor Co. F, -0.41% had to recall more than four million vehicles to fix a small component: a grommet that was part of the seat belt. This could break with repeated usage, making it impossible for the driver or passenger to lock in the shoulder harness. Each grommet cost only a few cents to make, according to a June 1972 Associated Press article. The recall affected most cars produced by Ford for its U.S. lines for the 1970 and 1971 model years, excluding only its Maverick and convertible models. The problem was easily corrected by replacing the original piece with another made of neoprene.
Here is the link to the biggest re-calls ever.
http://www.marketwatch.com/story/the-10-biggest-car-recalls-of-all-time-2013-02-16
David