550 HP should be easy. Just went through the same thing.
- Stainless Marine Manifolds (these were already on the boat)
- Rectangle heads with stock valve size and a valve job (matched to intake NOT ported) with Performer RPM intake and 850 Holley 80330 vacuum secondaries, one jet size down on secondaries
- Converted Gen V 502 Block to roller, honed only not bored -- Bob Madera (RMBuilder) Custom Cam
- Stock JE700 pistons with new rings, stock rods.
- 9.25:1 compression up from 8.75 (.010 off the block and .010 off the heads)
- 1.7:1 Scorpion Roller Rockers
539HP and 562TQ full dress with wet exhaust. Peak power was ~5100
I am sure I left some power on the table -- I specifically told Bob this was a family boat and drive-ability and good idle characteristics were the priority -- he is the man and should be your FIRST phone call, he was EXTREMELY willing to work with me to get the RIGHT combination. He is the cam guru but will help you through the whole process.
I fought this a bit when I almost went with a stock hydraulic flat tappet and I am SOOOoooo glad I talked to him. If you already have a roller motor it won't even be that much more expensive.