Please, please, don't get it started again. The discussion of steel vs brass plugs is so old it has whiskers.
I bought an engine (1957) cheap because the PO said it had an overheat problem he couldn't fix. I found the same build up problem with hardened stuff I suspect was cast iron rust mixed with radiator stop leak. Someone on Stovebolt said the rust build-up only happens on 216 engines. Thanks to John, we now have pictures. Anyway, I removed the side plugs, drain petcock and water pump to get access to dig out the water jacket passages being careful not to poke a hole into the cylinders. Not an easy task with engine removed. Almost impossible with engine installed.
The large plug hole at the rear has a shoulder that can break if you try removing the plug like the others. GM coated the outside of the steel plug with a hard shellac-like substance.
When you dislodge this blockage in an engine block you definitely don't want it going into your radiator. Disconnect the radiator hoses. Flush the engine out the best you can.
