Now keep in mind my car is 400, and is has a reproduced standard tank, and reproduced filler tube but this is what it looks like. I have a mech fuel pump at the engine going to a Holley 750 also.

Could it be the fuel pump is pressurising the tank somehow? The amount of pressure need to pop a gas cap that far seems greater than any amount of gas expanding naturally. Or can the smell be coming from the holley? when you smell gas next time pull over and check to see if the top of the carb is wet or the top of the air cleaner lid or in the air cleaner pan itself has wet gas there. It can be the float is set just a bit high and shoots gas up the vent tube....mine has done that.

I know I probably didnt help but just some thoughts.

NPD catalog has a cap like mine, part # C-3028-5, and Classic Ind has a filler tube sleeve that says helps seal off the neck and keep fumes from trunk and interoir

1967-68 Gas Tank to Filler Neck Hose
Seals inlet filler neck to tank stub neck. Eliminate gas
fumes from entering in the interior or trunk area when
properly installed. Produced with correct high quality
neoprene.
K630 1967-68 ..................................... 8.95 ea

Not sure how all this fits your setup since it's not stock