Howdy
I have had 2 gassers that would not run properly due to fualty gas caps. These were aftermarket caps that were listed in the book for my application. My advice would be to stick with a OEM cap and put a locking fuel tank door on the truck. As a side note, if someone really wants to steel your fuel they will punch a hole in the bottom and drain the tank into boat cans.
L8r
Conley