scott_fx:
the engine is an indmar (aka chevy) v8. i agree that it needs to be heavy duty. I'm hoping that the 200 amp versions would have plenty of headroom while cranking.
I think you're out by about a factor of five. I think 500A would be a reasonable guess for the crank cold starting current, but depending on the starter motor it could be higher. In any case you should design your components to have an ample reserve capacity. A factor of two would probably be sufficient, leading to about 1000A rating.