Maybe a helicoil job? What thread size is it? My Securon seat belt kit doesn't tell me the thread size

The seat belt has been moved down - see picture - but this is an uncomfortable location, so I'm told.[frame]
[frame]GeordieDave wrote:The L shaped brackets look perfect. Can you supply approx. dimensions/thicknesses please? I might be able to make a pair at my car restoration night class and save myself 40 quid!