[deezee]

It depends how your going to fix your seatbelts. 7/16th is industry standard. If you buy a belt with bolts on, they'll be 7/16th. If you buy a belt with clip on ends, then the eyebolts are going to be 7/16th aswell

You can buy seat belt stuff from these guys

https://www.europaspares.com/HARNESSES/NUTS_and_BOLTS/EYE_BOLTS__STRESS_PLATES__SEATBELT_NUTS_AND_BOLTS_ 2335.html