Fuel tank vent.

In my opinion the best way to solve vent problems is to duplicate the 69/ earlier vent. Look it up. Go to MyMopar and download a 69 service manual. Weld or braze a fitting into the removed filler tube so that it's far enough down that it won't interfere with a nozzle, and stick it in so it's flush outside, or nearly so. This is because you have to insert that tube from the outside.

Then just buy a length of (brake) tubing, and run it up high in the trunk, up near the rear quarter, and back down the filler tube and through the floor grommet, and end the tube down below the car.

Trying to use a vented cap my very well end up (on a hot day, with a full tank) with fuel running down the fender

It may also be that your's is not really plugged. There is a sort of check valve near the engine end of the tube. If you retain it, you want that tube up HIGH in the engine bay. Cut off the check valve, then see if you can get air through it