They measure flexible fuel hose [rubber, braided steel teflon etc.] by inside diameter and fuel line [steel, aluminium, SS etc] by the outside diameter. The gauge [wall thickness] determines the id.
My 68 is 3/8 od steel line to the pump and an AN-6 [3/8 ID] braided steel hose from the pump to the carb. I run an engine driven pump that supplies all the fuel I need to run 500HP at 6000rpm. Very quiet compared to an electric pump.
Although IF I was going to replace the line I would put in a 3/8 ID flex line the whole length.