Vapor return setup with 1970 "Evap Control System"?

Yes, that is what I am thinking.... extra line for the fuel return. Leave the vent line as a vent line to the front. IIRC, that '70 was designed to vent the gas tank fumes into the crankcase so that present line to the front IS the vent line.

And be careful on some of the parts store caps.... not all of them are vented even though they have what looks like a vent mechanism.