My 20' open has a 302 and when proped right and with a clean bottom it will top out at 32 KTS...
I have a 351 in my 26' Mack and it tops out at 25 kts.
I would go with a 351 on a repower but like it was mentioned anything over 30kts and it gets a bit snotty.
I have a 1982 19' Master Craft that has a 351 Pleasurecraft in it the 351 puts out 240hp. If I repower it will be a 5.7 MPI Crusader or PCM we sell both brands.I you want to go fast give me a call,We just got 7 Crusader 8.1 HO 425 hp in.This shipment is the last of them.