The only truly correct way to do this is to run the same size lines, fittings, everything from and to the tank. -6AN is pretty adequate for almost anything under 600 HP. Pickup in the tank if you can using an in-tank pump. Otherwise use an external pump near the tank, mounted lower than the pickup so it siphons naturally. Run that line to a good filter, near the engine. Then the filter output goes to an adjustable bypass fuel regulator. One (or two) output lines from the regulator go to the carb or EFI. Then you run a line from the return port of the regulator back to the tank. I have run this setup with both EFI and with a carburetor and it works great. I'm using a Tanks, Inc. 60 psi pump in the tank, and a Aeromotive Universal Bypass Regulator #13301, which is one of a very few that can be run with a carb or EFI (3 TO 65 psi). This is a key component to building a good reliable system, so don't buy a cheap regulator!
Sounds like exactly what I have done