Think the original is 5/16" line. If you had to choose between the 1/4" and 3/8", I would go with the 3/8". It won't appear to be any larger to anyone looking at it and certainly is easier to find than 5/16". If you use 1/4", it will look too small and will probably be to small to supply enough fuel for both carbs.
Just my 2 sense.
Joe
