Where to run return line on 67 Dart?????

Seems like a lotta work that IMO,from your description,won't solve a thing.
Think about it
All you are doing is moving the percolating fuel in the line to a new location. The float level will still be lowered. And the air filter housing will still be full of vaporized fuel.
But now you have the additional problem of no fuel in the fuel line, next to the carb cuz you let it percolate down the line.

Gasoline is made up of something like 22 different kinds of molecules,(edit: 200) each with it's own boiling point, and some of them below 100*F( edit 95*F). You need to figure out how to get your carb to run cooler than that, and stay cooler than that. Then you won't need to thro bandaids at a hemorrhage.
Gasoline - encyclopedia article - Citizendium