For reasons explained elsewhere, I would love to replace the 'spout' of
about ten Blitz five-gallon plastic portable gasoline containers with a
simple gas cap.

I called Blitz, who does not sell an alternative gas can spout - nor
would they answer this question.

Do you know what 'threads' these cans use and what 'might' be used to
replace the caps that would still handle the 22psi vapor pressure of
gasoline & remain air & water tight?