Does it have a Mercruiser ID tag on it anywhere? A chain-driven cam would turn the opposite direction than a gear drive, and that would require a totally different lobe shape and possibly a different firing order. Normal 250 Chevy gear drive cams turn counterclockwise when viewed from the front, while the crank turns clockwise.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!