What are you going to use for fuel lines????if it is the preflared lengths that are available at every auto parts store-----it will be an inverted double flare---according to what you have said you need a 5/16 and a 3/8----

if this is correct you will need a fitting that is 90 degree, 3/8 npt pipe thread by 3/8 inverted flare and one that is 90 degree, 3/8 pipe thread by 5/16 inverted flare

They stock these lines with the inverted flare fittings on the tube before DOUBLE FLARING and you will use however many it takes to run from the tank to the engine and back plus the connectors to join the various lengths---they come in 4, 5 ft lengths

If this info doesn't help you get what you need find a new parts store