Form242ps

Does anyone out there have any expierence with repowering a 1989 Formula 242LS or any year close to that. I just repowered mine this year with a good jump up from the stock 7.4 that was in it. I now have a stroked 454 which ends up being a 496 cu. in. , it produces about 460 HP @4800RPM and 515 FT. LB. of torque at 4200RPM. I was able to get up to 60MPH with the old engine now I'm getting 64 running a 25 pitch lab finished prop at around 4800RPM. This past weekend I tried the old 21 pitch Mirage prop I have and run up the RPM to about 5200 but lost speed , I was only able to get up to 58MPH. So now I'm scratching my head , is the 25 pitch prop good or could I get more if I went up in pitch. Does anyone have any tips for props.