Re: Petcock/Gas tank
All too many auto parts counter men today have no knowledge unless you can give them the make model and year of a car so they can look it up on their computer. Try another auto parts store, preferably an older one and look for a man with a few wrinkles to help you. If they have a fitting assortment, brass or gasoline resistant plastic, you should be able to find what you need. 5/16" to 1/4" reducing barb fittings shouldn't be that hard to find. I just browsed on Google and there were numerous pages to choose from.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"