I am running an Edelbrock ProFlo4 SEFI system in a 1968 GTO 400 4 speed. Using original fuel and vapor return lines are problematic with modern fuel. The lines must be shielded from all heat especially heat from the exhaust manifolds or headers.
The Edelbrock requires significant constant fuel pressure in excess of 55 psi for the horse power you are seeking.
(1) You can use 3/8 lines for main line and for the return. They must be adequately shielded from all heat sources or you will be vapor locked or flow restricted and your system will not have enough fuel under high rpm/torque demand
(2) Standard fuel gauge sending units normally have the "Vapor Return" line as a smaller size line and it is INADEQUATE to allow fuel to flow back to the tank and will cause a back up and prevent fuel getting to the injectors or throttle body. You will have to increase the size of the return opening on the sending unit;
(3) Holly makes a very good "In Tank Pump" that doubles as a fuel gauge sending unit and DOES NOT REQUIRE a return line. It has a built in means of keeping unneeded fuel from being sent to the fuel rails or throttle body depending on your type of EFI.
After experimenting with an external pump and fuel pressure regulator system using a return line, I found the Holly internal tank fuel pump to work best and cured all fuel delivery problems. No external regulator is needed and no return line is needed.
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