What year is the Zuke. Late model is an odd choice for an old 20' cc. Very heavy at 480+ lbs w/oil and a jack plate further throwing CG aft. Your boat was designed for about 300 lbs.
If you don't yet have motor, I'd rethink it. Otherwise, make the dealer work with you. Your gear ratio is like 2.50:1 so get the biggest wheel you can spin. Key to prop is stern lifting shape. Low to moderate rake and cup. Try a Merc Black Max in 15 X 21 to see how close you are to the high end of the recommended rpm at WOT. That prop is everywhere, is cheap and is a great stern lifter. You may need to go to a 4 blade to keep your stern up and minimum planing speed down.
It's an odd combination but maybe someone else has a heavy motor and can chime in. I'd borrow as many used props as it takes to get it right. Trim tabs will be very helpful. Consider them if you haven't already mounted some. Good luck!
