Marine specific blocks as far as I have been able to gather from the so called "experts". True or not , I couldn't positively say.
A marine engine holds water only running and pressurized by the pump, shut it down and it drains off like a leaky radiator. If they are winterized , you drain out all the water from pet-cocks on the side ,the hoses get disconnected and dumped ,pet cocks on the exhaust manifolds drain them. Anything left gets diluted with a RV antifreeze.
A boat you could get an engine from ,means no one cared anymore and chances are it wasn't winterized.
If he wants to save the 340 ,getting it sonic checked for core shift is a smart plan.