Thanks.
The casting has cracked at some point and was repaired. I was aware too much heat would cause a problem with cast iron. The idea was to heat the area around the plug enough so the Kroil would wick into the threads of the plug. I'd get it hot, cool with Kroil, tap with hammer, then spray again. Would do this a couple of times throughout the evening. The process worked very well loosening the radiator bolts with the exception of one where the head snapped off the second I applied pressure to loosen. Believed I would have that plug loose in a couple of hours. Well, that turned out to be a couple of hours for three nights. Haven't removed the plug yet, wanted to know if I was able to purchase locally at a supply store first.