One option that might be worth looking into is using the seat belts from another car.
I used the front and rears from a hyundai pony in my 69 Beetle to get the advantage of retractable belts. If I remember right, even the mounting hardware was metric and the same size.
These cars used to be quite plentiful in the pick-your-parts yards, but most have probably been long sence turned into coke cans or rebar by now.
Doug
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