It's major surgery time- - - -pull the intake and exhaust manifolds, and pry the freeze plugs out. That gives you better access to the bottom of the water jacket so you can poke, probe, and pry at the crud that's accumulated in there. Mix up a STRONG solution of Trisodium Phosphate and water and heat it to almost boiling. It's what I use to degrease firearms prior to doing a hot salts bluing job. Pour the solution into the block to the level of the bottom of the freeze plug openings. TSP is "concrete cleaner"- - - -industrial strength detergent. You can buy it at big box hardware stores like Home Depot. It will soften non-rust debris that acid won't touch. Use a turkey baster or a suction gun to siphon the TSP solution out of the engine and reheat it when it cools down- - - -it can be reused several times. Good luck!
Jerry
