I'm sure you have something similar in the US but just in the nature of FYI...
page 6 of 8 shows the mounting requirements for street rods
http://www.infrastructure.gov.au/roa..._COP_sec_8.pdf
refers specifically to 'seat belt bolts'
And in case you're having difficulty sleeping - a UN approved standard
http://www.comlaw.gov.au/ComLaw/Legislation/LegislativeInstrument1.nsf/0/FA508A2DCB831686CA25716300227BE4/$file/ADR+5-05+[FINAL+FRLI].doc
to save you some pain - page 30 of 81 has this line buried in it...
'5.5.1. An anchorage shall have a threaded hole of 7/16 inch (20 UNF 2B).'
LoBelly