7/16 unf as said is the std mount for pritty much every seat belt out there in the aftermarket and doner world
its been chosen for a pritty good reason as its what is the best for the job in the standerds of bolt sizes, yes you could use a metric equivlent somthing like m11x0.75 but thats going to be even harder to find. the iva manual states what must be used or "greater" (min of 11mm dia iirc)
mr iva tester isnt going to be to happy with you messing around with you drilling out the ends of a seat belts to a bigger size to get a bigger bolt in so your pritty much stuck with 7/16 unf.
also you will get the right fixings with the belt/harness.
if any one gets stuck i may have 6 or so nuts somewhere in the abyss of my workbench pile o sh1te
