It's going to depend mostly on what you want to do with the boat. If you know where the power peak will be for the engine, then set the impeller size so that the engine will hit it's peak. This will give you the highest speed possible, but may be trading some engine life. I suspect that your power peak may be something above 5200 rpm. If you set up an "AAA" impeller, you may not reach the full power potential of the engine.
A guy named Mike W is currently dialing in a blown 509 Chevy in a 21 foot Liberator tunnell. He hangs out at the RealJetBoats message board and website. Mike will be glad to share his information with you.
SteveM.