I have a 2004 33ZX with stock 496s she will run out at 75mph GPS, I noticed that 33 has stand off boxes with 600s she should run high 80s low 90s, She is a heavy boat and takes rough water well, big cabin "My wife loves it" What do you mean old school 600s? are they Mercury 454s with a blower or 500s with a blower?