I don't think the standard planks they used were anything special, my original 1980's planks were rough-sawn pine held together with steel straps and the tank sat directly on them. They lasted fine until Kevin Vernon cut them out when fitting the new chassis. He replaced them with similar.
Apart from the point about protecting the drain plug, I wonder if there may be a degree of vibration absorption for the fuel tank. I can certainly see no reason to change to a different material, what would be the point?
