Well, If your looking for that last bit you might have in there...
How many hours on the motors?
What oil are you using?
What rpm do you see at the top end?
Prop rotation out or in?
See if the blower shafts are splined or keyed. You might be able to drop a pulley size for a little more boost.
Make sure your blower belts aren't slipping...replace belts with new ones.
Fresh Plugs, wires, caps and rotors. Timing set to 36* at 2500. Check the drive alignment, see if it likes toe in or out.
Mark your trim indicators so you know were neutral is.
Try some 120* thermostats.
Water pump crossovers?
Crack the hatches open about 2 inches while at WOT...cold air baby!
Dead flat water is slow...look for a day with a 1' chop.
Make sure the flame arrestors are clean or even take a few windings out of them.
Good Luck and let use know what speed you pick up.

I had lots of fun with these...I miss them.

I will! Thanks for the response.