how neccesary is a return line for a electric fuel pump?

If you plan to drive the car with ethanol blend gas, in traffic, and pleasure driving? Absolutely yes you need a by pass. If your car is a drag only car? Then no. Dead head will be fine. I run a 340 sixpack in a 67 barracuda . Holley black pump. 3/8 fuel line to carbs. 3/8 fuel line back to the tank. Holds 6 1/2 pounds at all rpms. To 7000. Ask me why I did this? I had a Holley blue pump set up in a dead head system. In traffic I would get vapor lock and be dead on street until I could cool off pump. Remember ethanol blend gas starts to boil about 110 ish. With return system pump stays right around 80. Do what you want. I can only get ethanol blend gas where I live.