" 23 will drop you from 6800 to around 6k "

from 21 to 23 800 rpm drop no way.

I/ really think you are going to find you have bigger problems thn the prop. There is no reason a basic marine 496/430 motor should be spinning 6800 rpm. A 430 hp marine motor should not have heads that will do that, it should not have a cam and valvetrain that will do that, it should not have an ignition tht will do that,

Unless Bubba's back yard speed and chrome spec'd and built it, that motor doesnt and shouldnt spin more than about 5200 and have some kind of rev limiter Just setting a motor up to do that under load meas serious concessions have been made somewhere else in the powerband.

If that motor was built for a 22 foot boat pulling tubes and family, by 6k your valves would be floating better than a fat chick in salt water.

An interesting statistic, IF the tach is set on 6 cyl 6800 rpm = 5100 rpm with a v8. Just saying.
As far as I could find, the boat came with a 23 prop

2 inches of pitch is 9.5 % diff at 21 inches. 6800 times 90.5% is 6150.

that is a thunderbolt 4 ignition. no rev limiter. fresh motor. if new valve springs and done for any kind of performance application it could spin that high. what engine-prop you have in the 22 Avanti? what would it spin with a prop 4 inches too short? and that baja is a flatter bottom.