Gas Tank Vent

Look at the diagram in the thread. I BELIEVE the vent which originally went up front is in the TOP of the vent tube, and the ones going to the tank are in the BOTTOM. This allows the vent to be "above fuel" and allows any slopover to drain back to the tank. So I believe it's important to tie into the original front line.

Sounds to me like you likely have a pressure / vacuum cap. It should be rather deep and clumsy, and appear to have a valve inside.

The 69's and earlier, and some 70 (no CA) all used sealed caps

http://www.forabodiesonly.com/mopar/showthread.php?t=183956

Pressure vacuum cap in this thread

http://www.forabodiesonly.com/mopar/showthread.php?t=202184