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I certainly stand to learn more every day, and hope to do so. My comments are based around the standard Chris-Craft motor and manifolds, but I have recent experience with the more modern systems too. The NOS out of production aluminum Glenwood system I found in California was adapted to pre-heat water for the motor and I worked out the entire circulation system to do what I wanted it to do. It basically keeps the 327F flow system but with the updraft center discharge exhaust manifolds. I used a stainless steel block-off plate, and can't imagine how that would ever fail. I think the aluminum setup will need me to check the tightness of the long bolts from time to time, but it runs pretty cool (and fast).
Since I don't run in salt water I have no experience with it. My predisposition is to avoid salt in any hot metal because the corrosiion factor is increased, and to go with a stainless steel riser or final dump so you are not eating up cast iron (or aluminum). CC originally started with aluminum exhaust manifolds and I have a set of original 283 manifolds from 1959. Those would be long gone if they ever saw salt water. I guess it all depends on what type of exhaust system you have, what kind of metal is being used, cast iron or aluminum, or perhaps stainless steel, and how it is plumbed.