1965 Dodge dart..gas cap desperation!!! calling all experts!

Hey all! Finally got my dream dart....Taking baby steps towards a poor man restoration. My next step is a gas cap and OMG! Hubby and I have been all over the place looking for a cap that fits. Right now it has one that kinda fits and the seal is disintegrating. It looks like it might be a motorcycle cap. EVERY single cap we try from the parts stores (yes...all of them) have been too big. Even the ones that supposedly fit a 65 dart. So what's the deal? Does this year just have a weird size? I wouldn't be too worried about it excepting that it seems we are having some vapor lock issues....car will start and drive great then out of the blue just die (like it's not getting gas).
so school me please! lol
thanks!
ash
:prayer: