i upgraded my 5/16 to 3/8.

just need to get the 3/8 sending unit and block off the return port on it with a piece of fuel line hose a bolt and some hose clamps. Then you will need to change the inlet fitting on the fuel pump to 3/8". if you want, you can go further (like I did( and change the outlet fitting and line to the carbs to 3/8.

the new line goes in pretty darn easy. i was amazed because i thought it would be a pita. just remove the rear tires and jack it up and thread it through