From sitting out or due to water inside that was not drained ?
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My Olds did the same thing when I got, cracked from freeze plug to freeze plug. The boat was always garaged, so it was probably just water left in the block and the freeze plugs didn't do their job. I ended up getting another block and putting most of the old stuff that was still good and some fresh machining into it.
Did the engine run before you bought it? If so it might be worth trying to weld it up and see how it goes. I don't know how much you'd have to tear into it, but if it were to work, it would probably be the cheapest.
Just really depends on your funds. Good Luck.