from wikipedia:

Marine automobile engines are types of automobile petrol- or diesel engines that have been specifically modified for use in the marine environment. The differences include changes made for the operating in a marine environment, safety, performance, and for regulatory[1] requirements. The act of modifying is called 'marinisation'.

All of the "Big 3" American auto companies have had engines marinised at some point. Chrysler is notable, because the company marinised engines in-house through Chrysler Marine, as well as selling engines to third parties such as Indmar or Pleasurecraft Marine.

General Motors marine automobile engines are based on a gasoline truck engine. That means four-bolt main bearing caps instead of just two; sometimes the crankshaft is forged steel and the pistons an upgraded aluminum alloy. Most importantly the camshaft profile is different with the overlap ground to 112 degrees instead of 110. Expansion plugs are bronze to better fight corrosion. The head gasket's metal O-ring is also more corrosion resistant.

from marineengine.com forum:

"There are lots of differences when engines are "marinized", but the blocks themselves are nearly always basic automotive castings. In the GM small block, the 4-bolt main version of the 350 is preferable, but only slightly. The 305 isn't machined with a factory 4-bolt, but that's not a major issue.

Assuming your frozen block was not bored over, you'll need to find a donor block with up-to-spec original bores. Not easy to find in a junkyard, so you may want to consider boring a good donor 305 block .030" over and fitting new pistons. You'll need new rings in either case. Most engine yards will want to sell you a short block anyway, so you may not have to worry about fitting your crank to it. You may want to consider upgrading to a 350 long block - it's cheap horsepower and manifolds, electricals, accessories, etc. should bolt right up.

The reliability factor is determined by how good the machinist and engine assembler are (and eventually the operator, too!) If you shop carefully, you can put together a very good marine engine for much less than a new one.

Mike replied:

" Ok well lets see, are the pistons and heads marine specific? if not then a good quality 350 long block fitted with manifolds intake and exhaust and all the other goodies is all thats needed? on the other hand if the heads and pistons are specific to marine use then a donor block which has been line bored etc. could use the pistons also the cam. Which goes back to the 350 long block, I would need to install the nmarine cam from 305 or if needed a new cam? oris the 305 and 350 cam differant lift / duration?
Thanks for the help I am cheap and certified engine building mid 80's but no experiance on marine application. i would get a Very good machinist to do the work but I need to know whats up.........Thanks again Mike "

bondo added:

" mike,
Block is the same, change to Brass freeze plugs...
Heads, No difference.
Pistons, No diff....
the cam on a "crate" motor will have to be up-graded to a "RV" or "marine" cam....Low-end torque is what you want, Not high RPM horsepower..

the 350cid is the best way to go, All your "goodies" fit perfectly, you'll need to retune, rejet the carb. though... "

to which Kim wrote:

" Do not know about GM but Chrysler marine blocks were cast with a slightly higher nickel content to impede corrosion. Also we have to assume this is a standard rotation engine as auto engines do not come in counter rotating versions. Sometimes there were diffs in the oil pans and sumps as marine engines are typically installed at a slight down angle. The pans themselves were sometimes cast iron not stamped tin so they would hold up better. But generally speaking you can swap them out. Good luck and let us know when you are 100% again, Kim "

Need a Marine 350? We've got them