seat belt bolts

[68 ReNay]

what size bolt do i need for the seat belts in my 68? someone put after market belt in before and used different locations for the bolt. i bought a set of original belts (retractable outsides and one regular third belt) and i want to bolt them in the right, original spot but i don't know what size to buy. and it appears the inner belt bolt hole is a little wider than the outer one (having tried the largest bolt i had on hand in the threaded holes).

please help. i'd like to finish this this weekend.

thanks a bunch!