Upper Seat Belt Bolt Size 1973 scamp

hey folks!

i recently got some universal aircraft style 3 point shoulder belts for my 1973 scamp. Lower portions are installed, however I cannot for the life of me figure out what bolt size or how to mount the upper portion to the pilar. 7/16" bolt is too big for the factory hole, and 3/8" too small. I've also tried all the metric stuff too… Also how does it work with the housing pushing against the cardboard headliner? Does anyone have any pics of theirs?

thanks!