Is this a possibility for those stubborn woodruff keys (rather than pounding on the shaft system)? A person could take a tin snips and cut the old bottom plate out, snip it on the small piece between the shaft and the "half moon" cut out that is already there. It should slide off and you could remove, clean and put the new gasket back on. Then with a dremel or similiar tool, cut a notch (opposite of the half moon cut out) that would just let the plate slid past the woodruff key. A person would have to make sure it was perfectly smooth or even recessed down a little so the impeller moves across it smoothly and doesn't "chew" it up. Just a thought.
Allen