My 1971 flatback with original open transom runs with a 20" shaft 150 hp Evinrude Ocean Pro mounted on a 6" jack plate. The prop I'm running is a stainless SST 3-blade 14-3/4 x 17. WOT I'm nudging 40 mph and the hole shot is excellent. The prop runs with very little slip jacked up 3" and jacked to 6" slips more but will still get out of the hole easily. I see from the photos of your boat that you've raised the transom quite a bit, so a 25" shaft is probably just right. As light and efficient as these hulls are, I'd think a 3-blade would probably be your best choice.