you need the return line; without it, your float bowls can overflow, and you could have an engine fire. Besides, the increase in fuel flow helps prevent vapor lock. I don't think you will help vapor lock by using a rubber hose; indeed, I would think you will make it worse. ( I believe that a rubber hose would not dissipate residual heat as fast as a steel one) Plus, long rubber lines carrying pressurized fuel make me very nervous....the more steel lines, the better, IMHO. Especially near the fan..... Stock 240's had insulation wrapped around the steel lines, does yours still have it?
I run an electric pump full time and don't use the mechanical pump any more, but I use the stock fuel return line. So if I have vapor lock, the electric pump rapidly circulates cool fuel from the tank through the lines and back to the tank via the return. In other words, I don't have vapor lock. Running an electric alone requires some rewiring of the stock73 fuel pump relays; this is detailed in past postings by me which were kinda long, so find them with an archive check.73's came from the factory with both a mechanical and an electric pump, what happened to your electric?
Ghost is correct, you need the circulation back to the tank with a return line. The V-3 kit that I retrofitted back in the day included asbestos shieldng for the fuel lines and we also reamed the return line to a bigger size to allow more volume of fuel to circulate. I don't remember if it was specified or a trick we cooked up on our own. Good luck
My '72 doesnt use the return line. In fact, my overflow lines simply vent into the engine compartment. I know that's a fire hazard, but I've never seen them appear wet at the ends - and i've looked, believe me.