Thanks guys. Sounds too easy. So there is NO water going to the engine and head? Just antifreeze? No block to drain? Seen a flow diagram for a 3 litre but not for a 3.7 closed system. Would have a peace of mind then. Thanks again.
Yes the aluminum block on the 3.7 necessitated a closed system with AF in block , heads and manifold. NOLA told you how to drain the water side of the heat exchanger where the raw water pumped up from the drive cools the AF before it is dumped overboard via the exhaust system.
there are a couple diagrams in this thread
https://www.marineengine.com/boat-forum/threads/mercruiser-470-engine-consumes-coolant.471729/
There are a few guys on here who have run the 3.7 successfully, but it has a spotty reputation. Biggest issues are extreme sensitivity to overheat, if they get a little hot they roach the head gasket. So stay on top of your impeller maintenance and use OEM parts. Also make sure the heat exchanger is clean debris can get in the tubes. If you have the original 3" heat xch might be worth upgrading to the larger 4" one.
Other issue is circulation pump on front of the block leaking AF into oil. The pump is driven off the cam snout has double seals with a weep hole in between. Seals can wear a groove in cam snout so you need to repair or put a speedy sleeve over it when replacing seals
If the wonky magneto/voltage regulator dies, there are a couple of alternator conversion kits out there.
One issue you may face is some parts being NLA, cant recall if it is the exhaust manifold or elbow that people have trouble getting. They haven't made this engine since late 1980's or 1990 ish
