Re: Can you convert any engine into a marine engine?
Yeah, it ends up you can use the block (if you change the freeze plugs), the cylinders, the crank (usually), most of the head, lifters/rods, but not a whole lot else. Many boat engines started out as truck engines but the unique environment needs a lot of changes:
Shielding: Truck engines drop gas fumes out the bottom of the engine compartment, which is just outside air. The heavy fumes can't drop out the bottom of a boat, so it collects in the bilge in a cloud. Even when using the blowers, there might be some fumes around puffing out from time to time. That means everything electrical -- starter, alternator, various pumps, etc -- has to be shielded, usually with a fine metal screen, to keep sparks from escaping. The air intake above the carb is also shielded to prevent a potential backfire from igniting these fumes.
Pitch and Roll: Trucks may climb hills, but not like the sides of waves that boats take. The oil pan and pump have to be deeper so that the oil pump head is always submerged even if the boat is temporarily mostly sideways.
Power: Trucks and cars tend to like to run around 2000ish RPM -- transmissions can ease the truck up to speed, then drop into a lower RPM for cruising. Inboards like it around 3000ish RPM. The camshaft has to be different so that power/torque and fuel efficiency are geared toward the higher RPMs.
Cooling: The water pump on a truck is on the block. On an I/O it's on the outdrive, and the block holds a circulator. The exhaust system is cooled with water-jackets. Brass plugs aren't entirely necessary with a fresh-water cooled engine, but an engine compartment is a hostile environment, and if I operated in salt water, I'd be worried about paint eventually knicking off the plug and salt air corroding the steel.
It's cheaper and much less of a hassle to go with the marine engine, but I've read about a few people doing a conversion just for the fun of it. I've seen a few doing a Google search when I was looking for auto vs marine parts when I was overhauling my 3.0L.