sell the 454 and the 23p and buy a 502mag and a 26p
I had a similar issue with a 25 foot boat that did only 50 with a 350mag
swaped out the 350 for a 502 efi and bingo I hit a high of 70.6 on GPS
sure it was not cheap, but still less I though, if you have a buyer for the 454 your half way there