I hate to dowse this approach with ice water, but if there is so much oil reaching the combustion chamber that a plug is oil fouling, and indeed the oil is getting by the valves, I expect seals won't fix it, it will probably take guide replacement.
However, you should try to get Donnie's experience factored in here, I know that he has seen valve seal replacement accomplish a lot. But it seems to me that this is too much to ask. To fix it right, I believe you should pull the head, replace the guides and do a valve job. If the leak down shows no leakage past the rings, the complete valve job approach should result and a very good engine.