I'm running twin 90's on a 6.5M Mako. With 4 divers, gear and 12 tanks with a 12x21" prop i'm burning aprox. 8 gallons an hour doing 25 MPH @ 3,000 RPM. I can open it up it will burn 18 gallons an hour and do about 42. Empty the motors wil turn 5,600 RPMs burn 18 gallons an hour and give me 53-56 MPH. On that big of a boat and with 8 divers 16 plus tanks i would think you would need about a 17" prop and with that my RPM's max me out at 35 MPH burning the 18-19 Gallons an hour.
I'm running the newer fuel/oil injected Tohatsu's i hope this helps a little bit. If you are looking at a carborated motor then i can't help you other then tell you to at least double the fuel consumption.