The 280's you reference are most likely 400 cubic inch Chryslers (They also made a 383 ci version that was 280 hp). Fuel consumption will depend on how you operate the engines, trim tab usage etc. A reasonable number would be between 8-10 GPH per engine (which would give you in the ballpark of 1.3 gallons per nautical mile).
The 400's share a lot of the same components as the more popular 440's (so parts are not too hard to find). Boat US started stocking parts for the big block Chryslers as well.
I have a '76 Tri-cabin (one of the few years that they made the two station model). I've been rehabbing it (as well as living on it) since spring of this year.