I agree with Jeff. You could build some 460" (.030 454) with a small cam built for max rpm of say 4800-5K with a 750cfm carb on a dual plane manifold and it would definitely be more fuel efficient than the old ones you got and make more low end power for ease of getting on plane.
Unfortunately if you want better gas mileage, maybe refitting them with diesels? That might do it. Otherwise, maybe you should look into selling the boat and buying one with outboards.... the newer ones get pretty good cruising mileage... I think. Nothing related to performance boating is going to be fuel efficient. LOL