Serpentine belt drive for a 400

Back around 1986 the owner of a Ski Nautique got hospitalised from a motorcycle accident that caused him to forget he had not winterised his ski boat. It froze resulting in a destroyed block. I got the honor of the search for and replacment of the reverse rotation 351W. Found one at a boat repair and salvage shop in Guntersville, Alabama that had low hours. Other than some minor differences in the Holley carb for marine use, there wern't any visual clues I noticed that tipped you that it was a marine engine. The water log exhaust manifolds were fed from the heater hose position on the water pump using a "T" to send the water to both manifolds. The Mallory distributor gave no clue untill you pulled it and saw the backward cut of the drive gear. I suspect they changed the pump and front cover design as a continuing improvement after the model I was working on. The early versions did not like prolonged idling.