I am running a 1996 mark 5 454, I installed the holley300-42 intake, running 496mag exhaust on late model closed chamber vortec heads with stock cam,weber carb and closed cooling. the boat is a 24' excalibur with a bravo 1. the x demension is 2 inches above stock. spinning a 23 mirage plus at 4600 rpm. speed is 62 to 65. my problem is i have blown both head gaskets, port side between the rear two and the stbd side between the middle two, any ideas? timming is good and the motor has not overheated. when the engine was rebuilt the block and heads were checked and plained. the engine was fresh 30hours ago

What head gaskets did you use?

are the gaskets burned right through between the clyinders. If so it sounds like a lean condition to me.