How does your header spray look at idle and when running on plane? With those water cooler headers there is a common issue with water regulation and it seems most of the time too much water which gets pulled back into the engine. Also, too much cold incoming water touching hot engine parts can cause all kinds of problems so make sure your header water regulation looks good either before or after you look over the other gaskets. Don't get discouraged by this, you're gaining experiance and more reasons to play with your boat!