['68OrangeSunshine]

I got a gas cap from GMC Paul for 8 or 9 bucks. Tight fit no leaks.
Had been buying Stant caps from O'Reilly's and Auto Zone, but the rubber dried up and cracked after a while. I then got a tip from the forum that NPD had the perfect gasket, for 67-71 Chevys and Ford Mustangs for only one dollar. Fit was perfect, but the ethanol [+ whatever else] added to AZ pump gas soon wilted this gasket too.
Happy with the GMC Paul cap.
ECC [Evaporative Closed Circuit?] is emissions compatible stuff, came in w/ MY '71 or '72.
There's a return line from the fuel pump back to the tank, and a carbon particle fume escape vent under the hood. And vent holes in the Driver Side ''B'' Pillar.
Not for '68s. Forgetaboutit.
How is the shape of the filler neck grommet? If cracked, that can also import fumes.
I used to have locking gas caps. Sloshed a lot of gas over the side in hard right turns. Also lost a lot from evaporation when parked in the Desert Sun. I finally figured I'd been paranoid, and using the 'locking'' gas cap was losing me fuel. Not many theives have the time to siphon off gasoline. Too low a score for too high a risk. Millenials hate the taste of Regular, anyway.