If you're looking at a block that's been in salt water & used an "open" cooling system (pulls in water from outside the boat, circulates it, & spits it back out) be VERY wary of water jacket rust. I've personally been thru 2 marine blocks (one a 413, the other a Buick v6) that rusted thru the walls from the water jacket due to being used in salt water for many years.