Merc use's good stuff. I have built 2 sets of 383's putting out no less then 485 hp,...on pump gas. Use the best heads u can afford...be sure and have good comunication when buying and or having work done. In other words tell them you RPM range, and thats its for a boat obviously.
The only other motor I would build other then a 383 would be a LS2 punched to a 402. Sweet
Have fun with the build