[Brian]

Do not plug the overflow tube!!! It is there for a reason; as the coolant heats up, it expands. The overflow tube allows this excess coolant to escape from outta the top tank. If you plug it. the top tank will explode under the pressure build up. What I do is run the overflow tube into the bottom of a container, open to the atmosphere. This container will catch the excess as coolant heats/expands, and will allow the coolant to siphon back as radiator cools. ala recovery system. ...as long as your radiator cap seals