Mike, you're talking to a guy who every year about this time, has difficulty knowing if he should advance his watch one hour, or retard it one hour, so beware of the info, lol. I'll offer what I can, but you should get another opinion before twisting away on that distributor.
The LH and RH distributors turn the same direction, due to the gear on the bottom of the distributor shaft making this happen.
I can almost feel it now if I close my eyes, but it's been a while.
Once the motor is wired up to a timing light, it becomes readily known if rotation one way or another is an advance or retard situation.
Looking at these diagrams, if you look at the rotor alone, and sense the fact that it is what is turning, look at cylinder number one and you can see exactly when that cylinder will fire. If you turn the distributor housing to the left (counter clockwise) that would appear to take longer for the rotor to reach it's target point at cylinder number one, and I would call that a retarded situation. If you rotate the distributor housing to the right (clockwise) that rotor gets to the target sooner, which I would call an "advance".
I've hot rodded a variety of cars in the past, including some exotics from UK, continental Europe and Scandinavia. Once you change the cam profile, compression ratio and carburetion, then all the factory settings are thrown to the wind and you're on your own. I would "tune by ear" many times and rotate the distributor ever so slightly one way or another to find the stumble point for the retard and the advance, and would set a middle baseline. Then I would advance all the way to the stumble point and back off a bit, and take er out for a spin, would read the plugs, set the carbs, and do a speed trial. It worked for me then but it never seemed to work very well on a stock marine motor.
CC motors are to be timed in gear at 500 RPM (I always was a little faster than that), and I could never beat the actual factory timing mark any better by ear. Therefore, I recommend going right by the book on the timing mark. On the 427 it's on the flywheel under the pointer. Naturally if you're out on the water under way, you'll need to have someone aboard doing the steering for you, and please secure those engine bay hatches, they can be downright dangerous if they slam down on your head if the boat rocks!!! People have done this at the docks, but it puts a strain on your hardware and the dockmaster probably won't like you very much. It has a tendency to kick up all kinds of silt in the harbor and it also has a tendency to put a lot of stress on the dock structure too.
It's probably best done on a quiet stretch of water with someone at the helm to give you a hand.
See the note to COMPLETELY separate spark plug wires #7 and #8. Time the motor so the dimple on the crankshaft shows up under the pointer at 500 RPM.
Hope this helps,
All the best,
Paul