I soaked top and bottom threaded section of the rod with PB plaster for a few days. wrap some tissue around it and keep it soaked. keep a rag underneath as that stuff drips and smells. Loosen the shrouds above with the turnbuckles. Port and starboard. Then I think I wrapped some tape around the rod to protect it and clamped vise grips on tight and used a large wrench on the bottom nut. Use either a socket or Combination wrench whichever fits. Wear gloves to protect knuckles. Might have to use a hammer on the wrench to break free from the rust. If it is so bad that it will not break free because it is a big ball of rust, then sawzall the rod in pieces as the threads will have been compromised to the point of needing replacement. I soaked the nuts and washers in almost straight ON/OFF after I removed them and everything shined up very nicely.  Garhauer Marine makes all the chainplate hardware or at least makes the replacements.