I'd stay 451. Reason being is more meat to dissipate heat. You're gonna need one hell of a cooling system. I know my grandfather is having an issue with his boat right now. It's a Merlin inline 6 and it runs very very hot. It's all about air flow. One thing too I'll add is you might consider adding a fuel cooler. We had too and this was a fresh factory boat but it kept vaporlocking. Fuel cooler stopped that.
