I also have a 65 dart w an aftermarket locking gas cap. I have on occaison heard a whoosh when I take the cap off & the only time I run into a gas starvation problem is when I inadvertently let the gas gauge needle get down to the left side of the E so I am assuming that I am OK. Pull the vent line off off the filler neck inside the trunk at the short neprene connector & blow thru it.

live every 24 hour block of time like it's your last day on earth