For a big quality closed cooling heat exchanger tank for those motors I would contact the guys at Keith Eickert. They have some big stainless units that can be polished and stringer mounted that are really sweet. I would advise running a thermostat of 140-160 degrees depending on your average boating water temp and make sure you engineer your raw water plumbing carefully to control both flow(quantity) and pressure (force) and proper sequence of coolers on the raw side. Usually you hit the oil cooler first, power steering, transmission or fuel and then hit the heat exchanger last before you exit out thru the exhaust manifolds or headers. It may even be advisable to split the raw sytem at a tee and hit the coolers of one side and the exchanger off the other side of the tee. Kieth Eickert or any quality rigger should be able to set you up with the correct plumbing and some tips on a sano installation. Good Luck.

Ray @ Raylar