The previous guy put an aftermarket sender in with an extra fitting. There WERE NO vapor return systems except for the street hemis and the ??70 sixpack engines.
The emissions carbon can system is a different system, and those were not installed until "some" in 1970 (California)
ClifCuda is right--the vent comes out of the filler, goes up, back down, through the grommet, and ends right inside the frame rail.
This 66 Plymouth manual does not cover Barracuda instrument panels or other "special" Cuda stuff, but here's a 66 manual: