I had a similar overheating problem with the original 216 in my truck back when I restored it in high school. Progressively got worse until I couldn't drive it around the block. Never could figure out why. 19 years later after university and starting a new career and finally having time for the truck again, I pulled the engine to paint it, and found a leaky frost plug. When I pulled it the water jacket was full of crud up to the bottom of the frost plug. I'm not re-installing that engine anymore, but after reading this post, it reaffirms what I thought was the cause.
I ended up having the block hot tanked to clean it out before deciding to go with another engine, so didn't get a good look at the crud, but my grandfather had put more than one jar of bar leaks in there. Interestingly enough, that wasn't his go to for radiator leaks. He swore by a handful of pepper to stop a radiator leak.
