The Merc 170 was also called the 470 and 485.
Not merc's best motor, just slightly better than the OMC 2.3 liter
Read up on cam bearing sleeves, alternator conversions, modifying a ford 460 head, etc to keep it alive. Hope you have the upgraded heat exchanger and exhaust manifold.
Aluminum block, steel head, with floating bores is a sure-fire leaking head gasket
Soft cam with seals running on it is sure-fire coolant leak