Datsun Roadster Forum-311s.org

If I remember, the bolts are 7/16-20, about 1 inch plus the thickness of the seatbelt tab. Some have about a 3/8 un-threaded shoulder and an oversize thin head, so the tab can swivel. Google search 7/16-20 seatbelt bolt.
For my first roadster, I got surplus airline seatbelts on Ebay, they clipped onto heavy duty eyebolts.

I kno good & w this is an old thread, but I just bought a set of 3-point belts from seatbeltsplus.com. I am getting ready to install them now but they look pretty inexpensive.

Peace,
Pat