Seat belt bolts tighten against the shoulder, or the ends of the threads, pre metric are usually 7/16 fine thread, grade 8 bolts do work (field tested in dirt track race cars). The length of the shoulder depends on how many belts use the same bolt, or how much spacing is required for the belt(s) and whatever spacer may be required to clear things that may damage the belt webbing (think inside plastic trim, or other belts). If more then one belt uses the same bolt, the belts both have to be able to move freely, independent of each other, the webbing of the belts can not rub against each other.