I have a marinized chev and normal 350's in my cars, the difference between the marinized and the street units are usually as follows, camshafts are different due to the torque being required at 3000rpm fixed, the sumps usually have no drain plugs, welch plugs will be made of stainless steel to stop corrosion, water pump will have stainless impellers, intake valves will be stainless, all the add on bits are usually either cadmium plated or replaced with stainless and then lots of paint, as far as running backwards are you sure, the reasdn I say this is because your stern drive legs that you attach to the engines have a lever which modifys the leg to spin left or right not the engine.
So all up to go from marinised to street just a new camshaft, to go from street to marinised expensive.
Steve