Fuel line size?

Hi - I have 1963 Valiant 200, with a 225 slant 6, my question is the fuel line size. I was looking to replace the fuel sock in the gas tank, but I understand that there are two sizes of fuel lines used, 3/8" and 5/16", how do I know which size I have???
IF you say measure it, is it the ID or OD?
Thanks