I suspect your under powered anyway with only 310 ponies for a 25 footer and that you're probably in the low 60's. If you mean HP500 that's 470 horses and that would (I'm guessing) put you in the mid to upper 70's at least and would require re-propping as well to optimize for the boat. That's also a lot of dough to drop. Have you considered trading up into a boat warrantied with a HP500?