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There was a thread on this just this week, I believe. Do a search and you should find it. In the mean time, try an adjustable wrench. That works on my '76. I don't believe there is a socket that fits it.
You can pick up a set of hex wrenches to solve the problem. They're not expensive and will come in handy for other projects. I don't know whether that is the original plug or not. In any event, clean all that crap away from the plug prior to removing it.