71 Dart, Fueltank to crankcase vent as return line?

Not sure but I think you are confusing terminology. "Crank case vent"

Doesn't your car did / does have a vapor separator in the trunk?

That should have a 1/4" line coming from that up to the front of the car.

"You could" use this for fuel return with a few caveats.

First that one single 1/4" line IS THE TANK VENT. This means you'd have to do "something else."

The minor caveat is that line has a little valve near the front end of the line. You would want to cut that out of there

IF you do this, here's what I'd do. Pull the filler tube out, and weld, silver braze a fitting up high on the filler neck and then duplicate the 69/ earlier vent system.

This simply is a 1/4" line coming off the filler, going up high in the rear quarter, then coming back down along the filler tube and "open ended" inside the rear frame rail.

Here

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

The early vent system. It's simple and works