At this point I think all that is probably left is at the bottom of the water jacket, so to keep the chemicals to the minimum I would plug the lower holes and put the chosen chemical in the block and let it work for awhile, drain, rinse and then hit it with the pressure washer. If you have some of that crud in the picture left put some of it in a container and see what happens with a couple different chemicals. I have had good luck with muriatic but it is dangerous stuff. I like the lye idea and may experiment some with that. We used to use lye to get the butterfat out of milking equipment years ago, I had forgot about that.
I replied to Sput because that is some really useful information.
