3/8 vs 5/16 fuel lines

I gotta ask why you are running both a mechanical and an electric pump and not one or the other!? We run electric boost pumps on some of our aircraft engines, to insure fuel flow in high flow / nose high attitudes in case the gravity feed mechanical pump loses head pressure, but they need to be correctly check valved and/or be a proper flow through design to work properly. Many electric pump designs don't allow fuel to be sucked through them (if they aren't running) by the mechanical pump.