Went through a similar situation with a friend's boat last year. He bought a 19Ft. early 90's open bow with a 4.3. Got is cheap. Owner said it had a cracked-transom, which it did. He "forgot" to mention the engine had froze over the winter and was cracked everywhere. It ran fine, but water leaking out of every pore when it warmed just a little. We had a hell of a time finding a decent core. Marine cores were VERY expensive. Ended up buying an engine from as Astro Van with pretty high mileage but clean. Changed out all the parts, and new gaskets. Put it in the boat. Ran very quiet and thought we were done. After about 10 hours of time on the water, out of nowhere, it developed a lite rod knock that became a LOUD knock after about 15 minutes. Ended up doing a proper +.030 overhaul on the Astro engine. It's been fine for hundreds of hours now. My advice is to rebuild the new engine properly, using the rotating assembly and heads from the new engine. You can save some money by possibly using the marine heads over, but absolutely have them pressure tested. If the block froze and cracked, chances are very good the intake manifold, exhaust manifolds, crossovers, heads and anything that has water going through it is also cracked. Going to need to check all that stuff. Either magniflux or better yet pressure test. If you keep the RPM's reasonable and have the block line honed, a 2-bolt main 350 is probably fine. Both the 4.3's I spoke of were. If the marine cam is OK, I would reuse it in the new engine. Our marine cam was chewed up from engine failure and the engine was down on power a little with the Astro Van cam. Good luck with it.
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