Where to get replacement 1972 5/16" vapor return line?

The vapor line must come up high in the bay, and it has an anti siphon/ check valve at the engine end to prevent the tank from siphoning on overfill or if parked steeply downhill. Download yourself a service manual from MyMopar.com and get into the emissions and fuel sections of thebook

Also we have a lot of guys trying to insist on using them for a vapor return. That line is the VENT and if you use it for a return, you must do something else to keep the tank vented. The caps--NONE of them, are vented. The evap tanks have what is called pressure/ vacuum caps, which vent only for pressure or vacuum release, like relief valves. They themselves are a nightmare as they have changed pretty much every year.