if you fill the water passages with filler, I understand how that strengthens the block, but how do you then cool the block? How else is the engine getting cooled if you're not running water through the passages? I've never understood that.
Normally the block is not completly filled. My 428 is filled to the bottom of the water pump holes, about 1.5 inches from the top of the block. Some people fill a little higher but there is still room for water to flow.
Understand block filling is normally a drag engine only deal though people can do some limited street driving with a filled block.