I am building a 547 Mopar jet boat. My first idea is you need to try out a vacuum gauge to see if you are pulling vacuum at wide open throttle. I think any big block buzzing along at 5000 RPMs is going to be consuming more the 600CFM, I think 700-800 would work alot better. If you decide to try a Holley, check the vacuum at idle and make sure the power valve is set for the engine vacuum, the number on the power valve should be half of them manifold vacuum is at idle. I have never used a Predator, but they are supposed to work since it is a variable venturi carb. I am using a Jacuzzi jet (eek) with my motor, I will be using a stainless A impeller
if I can get one, if not American Turbine makes parts to change it to use different impellers. We'll see. Pictures of my boat can be seen on my profile. Brian