OK, gonna try Jerry's TSP first, then probably also use a coolant system cleaner. Going to try to clean just the rear area of the block first with the plug still removed so I can still see in there to determine how well it's working. Then install the plug and fill the whole block.
My biggest concern with the TSP is a cold engine block in the winter sucking the heat right out of the water fast. Especially a small quantity in just the back, but I'm gonna give it a try. I'll replace the water often.
