One bolt holds wheels on cars (Hub bearing ) !!! Propellors on planes; UJ`s to box`s ;propellors on boats; seat belts to anchorage point;climbers on walls. List is endless where one bolt suffices perfectly adequately !!!
Put a lock washer on your fin bolt or tighten it up correctly !!!!
Good luck with conversion !!!! You could always fibreglass the fin in just to make sure !!!
Why are you putting spacers in a PB ??? Very unlikely they are needed !!!