550 horsepower at 5000rpm will require a larger engine than a 460 so adding stroke and cubic inches is the way to go. I think you could reach your goals with an engine that's in the low to mid 500 cubic inch area. About 4 years ago I helped a guy build a 454 Chevy for a jet boat and it made 509 horsepower at 5200rpm with a set of mildly ported oval port cylinder heads so with some more displacement and a better cylinder head you'll be way ahead of his engine. In his build he used a Competition Cams street roller cam, I'm thinking that it was the 280R which worked great and it's caused no problems with the valvetrain. A single plane intake was worth 30 horsepower at the peak and it was much better everywhere we tested from 3000 to 5500rpm. For a carburetor we tested with my Competition Carburetion built 950HP and his 850 Holley. After some jetting they both produced the same numbers and cruising down the river it's flawless.
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For that low of an rpm range i would go with a 4.5 stroke for sure, a mild roller cam and some CJ style heads with a Victor intake and 1050 carb. For what you want this will get you a lot of low end power for your dollar. 
So what's your projected cruise rpm ?
edit:
Do you ever plan on running it really hard? What intake manifold and carb do you plan on running ?
G-
Been there done that many yrs ago, if I could do it over again I'd take tha mph up top and give up alittle on the cruise side.
It's a balancing act,
Years ago we ( brother and I) had a stought roller cam 472" Ford in the CP with a AA impeller. Engine broke because of abuse of the little button on the steering wheel, replaced that engine with a nearly stock 460, that 460 damn near used twice the fuel of the more powerful engine. It didn't have the power to turn the impeller so it took more throttle effort and used more gas.
I don't think I see 2,700 rpm being a realistic cruise rpm, the impeller is only going to move so much water. I've had a 9.25" AA it pulled like a AAA the 565" turned that impeller 5,200 and the boat ran 92-93 mph and 102 at 6,200 on the Nos. I wish I'd have let the engine rev out to 5,800 with that impeller. That pitch impeller is no longer made and it was the most efficeint impeller I ever had ( mph/rpm).
I don't cruise the CP as most people run there boats. Most people cruise at 3,000-3,200 kind of lugging the boat along I run my boat fully loaded up river 12-14 miles at 3,800-4,100 at 50+/- mph on top of the water with far less drag. It doesn't use anymore gas until the secondaries kick in.
I think an 850 carb might be a bit small for your engine. Here's another thing on the carb, since '93 the 565 has had the same 1,050 Holley Barry Grant carb. First the engine was 780hp, then 855 hp and now 935 hp the same carb with progressive linkage, the secondaries didn't open until 4,000 +/- depending on the laod in the boat. Last year I got a 1,200 cfm Pro-System carb with soft-progressive linkage (secondaries open sooner). Patrick told me because of the way the carb is balanced it won't use anymore fuel than the other carb, as of yet I'm not sure about that.
I can't write you all that I've picked-up since '89 trying to balance cruise rpm with performance, but I agree with what Filucky stated about giving up a 100 or 200 rpm in cruise and picking it up on the top end. I'd let the engine rev out a bit more.
Do you plan on putting this engine on the dyno?
More later but I have to go for now.
G-
Jon
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