I have had to do this a couple times with the offenhouser manifolds, I tried the steel washer with silicone on both sides and the rubber washer and still had them leak. This is a great help with the problem of Electrolysis, eating away the metal. The main thing to remember folks is never put tap water in your radiator, always add antifreeze in a 50/50 solution and bottled water, because of the chemicals, and minerals in tap water. Different parts of the country have different minerals so all cant be covered. On Johnny Macs Motor, I would put the bolt there for show.
Kevin