Adding additional heat exchangers

[Keith]

I have 8-71's and would like to add external heat exchangers to both engines for two reasons. First, I want to keep my engines warm during the winter while running my Webasto heater system. The Webasto circulates fresh water and anti-freeze as well through its plumbing system. Second, when I am running the engines the reverse would happen, I would use the heat generated by the engines to transfer to the boat's heating system. And, last but not least, I want to make hot water underway since I have a 20 gallon hot water heater with a heat exchanger built into it. My question is - can I tap into the exhaust manifolds? On my engines there are a couple of extra plates on the out side of the exhaust heat exchangers. My thought was to remove the plates and drill and tap them for NPT thread which would become my source for circulating water. Has anyone done this before? Or is there a better place to access the fresh water side of the cooling system?