The biggest issue I have run into with 216 and 235 engines in original condition is not rust. It has been what is very likely lots of stop leak products put into the cooling system to seal a leaky radiator.
Back in the day (when HRL was still an old man) labor was cheap, but parts were expensive. A brass radiator must have very expensive then. Stop leak was cheap and easy.
That caked up black stuff can be miserable to remove even when you are down to a bare block on an engine stand. I doubt that there is a chemical known to mankind which will dissolve it without dissolving the cast iron block.
