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.