Vent tube on Gas Tank Sending Unit?

That is your tank vent. .

NOPE NO FACTORY CAR in those years had a 2nd fitting on the sender

OP: Go to MyMopar and download yourself a free factory manual. Page 14-75, an example:

vent.jpg

This basic system was in place up until the "some of" 70 cars with the CA evap emissions controls/ carbon can setup, and Federally thereafter, which used a separate "vapor separator" can for a time, and later, a tank with the evap separator built into the tank, and multiple upper tank fittings.