I am not certain about this, but I think that leaving that hardened crud in the bottom of the water jacket leads to the classic crack in the water jacket scenario. I had a 216 develop that crack in the middle of the summer when my original type brass bellows t-stat decided to stick in the closed position on the highway, causing the temp gauge to peg. Upon tearing the engine down, I discovered that the horizontal crack and the top of the crud inside of the water jacket where at the same level.
Coincidence?
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission