1975 dart sport gas tank vent blocked?

The tank vent has nothing to do (or very little) with filling the tank. That is "done" by the baffle in the filler, where fuel is supposed to gravity run down the lower part of the filler, with the baffle allowing air to come back up the "upper" side. You will NEVER fill a tank with a 1/4" vent

The ONLY vent on a 71/ later which has the evap emissions is a 1/4" tube that runs up into the engine bay, and interconnects with the evap/ carbon can system. The filler cap is NOT vented as the term is usually used. It is called a "pressure vacuum" cap and is a pressure release for build up in each direction, AKA too much pressure, and too much vacuum.

As far as the vent into the tank, I do not believe there is any check valve there, but might be. There normally is an "antispill?" "anti siphon" ball in the 1/4" line up in the engine bay. Go download the 73 manual (latest you can get free) and read up on how it all works. Look in the fuel section covering the tank as well as the emissions section towards the back