There are three completely different exhaust manifolds commonly found on 4.234 marine engines - the North American (cast iron), Euro (aluminum) and Bowman (aluminum combination manifold / header tank and heat exchanger). The North American manifolds corrode and scale badly in marine environments. Eventually the wet side either clogs enough to restrict raw water flow and overheat the engine or they rust through, dumping raw water into the exhaust side. I have seen photos of aftermarket stainless manifolds - they look beautiful but I believe they still run raw water through the manifold and still use the factory heat exchangers and header tanks. There is a better alternative - I recommend converting to a Bowman, which will replace your current exhaust manifold, header tank and heat exchanger (which are often at the end of their lives anyways). Raw water only touches the inside of the Bowman's heat exchanger bundle, and the manifold is cooled by engine coolant as it passes around the bundle and throughout the manifold. These are currently in production and readily available, and they are the most cost effective cooling system to maintain. Depending on your setup and application you may also require a different intake manifold, along with plumbing, gaskets, etc. Trans Atlantic Diesels sells a complete 4.236 Bowman kit with installation instructions and (I believe) a DVD.