Just a good freeze with no antifreeze can produce that long crack on the lower rear driver's side.
I have a 235 with that clean and long crack sitting in my garage. It came to me cracked (and, unfortunately, is the "matching-number" H-M engine for a spare Hydra-Matic transmission).
I can live with that kind of crack. I put 25,000 miles on one without any issues. Mine cracked on a hot day when the original brass accordion-style t-stat decided to stick in the closed position, causing a major overheat and subsequent crack (I think). It needs to be mentioned that the water jacket was later found to be clogged up to the top of that crack with petrified dinosaur dung. Did that cause the crack? I don't know.
The OP was talking about a crack "almost the entire length of the water jacket." A different animal in my opinion.
Carl
