The switch from 3" to 4" is the heat exchanger not the manifold. If it has the conversion for the alternator and the points to electronic ignition the only other major worry with the engine is the aluminum exhaust manifold to the cast iron riser/reservior. This is were the failure takes place and dumps antifreexe into number four and to an extent #3 cylinders. As long as you check the reservior and exhaust manifold connection every two years you will not have a problem. this is a very good engine with an undeserved bad rep. It has the above issues but if you stay on top of them it will run for a very long time. It is also very economical.
I ran one in another boat for 8 years without a problem. Only changed oil, plugs, and the exhaust/ reservior gasket (every two years) and that is it.
Parts are still available. I have a complete spare engine in my shed if you purchase it and need parts. My spare does not have the conversions.