You know, after thinking about it, you would not even have to submerge the entire block if all you want to do is clean the inside. If you can get an anode or 3 inside the water passages, just seal the block and fill it with electrolyte. That way it will not be able to affect the machined surfaces. Just leave the top open so you can let the gas escape (oxygen and hydrogen) and to add more electrolyte as needed. Obviously, you can't let the anode touch the block.