7/16 fine thread is a common international standard. Not sure of the lengths though. Typically they say you want at thread engagement equal or greater than the bolt diameter. Get them long and cut them down. If the fine thread is not avail at the hardware store you can order them from racing supply shops online and get them fast.
I'd just pull a seat belt bolt out of another car and see if it threads into the cruiser. I recently ordered belts for my 81 and they came with 7/16" bolts.