To save a lot of problem with the return line and which tank to send it to, just dump the returned fuel back into the feed line by the tanks. It will recirculate the fuel without filling either tank, and being back away from the pump, it should have time to cool off.
I would think you are going to need a regulator at the carb to regulate how much fuel returns and how much is going to the carburetor.
Here are some links to fuel regulators and how they are plumbed. The first one is a vacuum operated type which would work good for you. They return more with high vacuum and less when vacuum drops like under a load.
http://www.911chips.com/preregan.gifhttps://musclecardiy.com/wp-content/uploads/2015/07/173.jpghttps://www.thirdgen.org/forums/att...4-l69-fuel-return-fuelpumpcircuit01a.jpg
