Several dealers sell marine conversion equipment for auto
engines. I once had a converted Chev 283, and the differences were
these:
1. Water-cooled exhaust manifolds. The closed-in type of engine in a
boat means no airflow around the exhaust and they would get red-hot
and cause a fire hazard. The cooling is via jackets in the manifold,
and the water leaving the engine is also dumped into the exhaust
itself to cool it further and muffle it some. Only in wide-open
installations do they use non-jacketed marine headers, but they're
still cooled by the heated water leaving the engine, which, as with
the jacketed system, is dumped into the exhaust manifold to cool the
exhaust itself.
2. You might need an oil cooler and adapter to direct oil from the
filter mount though the cooler and back to the filter. No airflow
around the oil pan, see, and a hard-working engine that generates a
lot of heat. The transmission, if it's a hydraulic type, will need
another oil cooler.
3. A positive-displacement water pump that draws water from beneath
the boat. The centrifugal pump found on auto engines won't lift the
water unless the system is pre-primed, a real headache, and if the
system gets a little air in it the prime is lost again and the engine
burns up.
4. The thermostat housing is changed to one with a couple of 3/4" hose
nipples, and the water pump cavity is closed off with a cover having a
similar nipple.
    So the water is drawn from the lake, through the pump, through
the oil coolers, divides and runs through both exhaust manifold
jackets, out of those and into the water pump cover and through the
engine, out the thermostat housing and into the exhaust manifolds
where it's blown out with the exhaust.
5. If the engine is mounted at a radical angle as with a straight-
shaft setup, the carb must be mounted on a shim to level it.
6. Engine mounts are rigid if a straight-shaft setup. The shaft is
bolted rigidly to the transmission's output flange and the engine/
trans can't be allowed to move around.
7. A marine transmission, probably the most expensive proposition of
the whole works. Most of them are forward/neutral/reverse, using
clutch packs similar to those found in a car's automatic transmission.
The thing is really stout, much stronger than an auto tranny.

        Get ready to spend lots of money to do it right. See
https://www.boatdesigns.com/departments.asp?dept=35

              You also need a prop shaft, prop, strut, stuffing box,
and on and on.

               But it's fun.

               Dan