finally got the bugs out of my restoration on 330/ 454 build up. Tested this last weekend and ran 62 mph at 5200 rpm with 4 people on board. Started with a 330 peanut head 454 in a 2550 sport Chaparral. The boat weighs about 4800 lb dry. Kept lower end of engine stock. Swapped peanut port heads for a # 290 large oval port off 70 corvette, (262 cc runners) and closed chamber. Figured the compression to be about 9.2 to one. Added Crane hyd flat tappet with about 519 lift on intake, 3/8 push rods, Crane 1.7 gold rockers, Edelbrock performer rpm manifold, Holley 770 cfm Street Avenger converted over to marine use. The boat used to run about 50 mph @ 4500 rpm spinning a 19" Quicksilver prop.

Swapped prop to a 23" quicksilver Mirage. I'm running the stock exhaust manifolds. The holeshot is not as good but makes up for it on the top end.
Question: How much HP is the engine putting out? How much would aftermarket exhaust manifolds help? I'm happy with the performance but a little more wouldn't hurt. I know im pushing my luck with a stock bottom end.

Sounds like you've done a lot of work the get air and gas into the engine ( nice results too) and nothing to get them out. Are you running thru hull exhaust? You should at least do that. You may find another 100 rpm by adding aftermarket exhaust.
But, I've been told the stock 330 bottom end doesn't like to spin past 5000rpm. You may want to add some cup to your prop. That won't affect your holeshot but will increase speed/lower rpm in mid range and top end.
As far as HP, it seems you are better than the 454mag maybe 400-425.?

stock bravo exhaust. The Built in swim step is in the way for a through hull setup. I could exit them out the side i guess.

Go to go-fast.com then "knowledge base" for HP calculator. Your bottom end will be fine if like Payton says , you don't over rev. Watch out for to much timing with the cast pistons. I'm making about 450hp with more cam(and roller) but smaller vortec heads.

The weakest part of your bottom end is your rod bolts. With good ARP bolts bottom end will be good to a least 500 HP, if every thing is straight and balanced. Quality machine work and assembly is the key . It sounds like you are making about 400-425 HP.

I agree with the 425 or so hp,my boat stock with fuel was around 4800 lbs and ran about 62-63 w/23 pitch with 415 hp 502 at about 5100 rpms,Your doing pretty good for what you started with,Smitty

thanks, Smitty. What type of exhaust manifolds are you running? I would like to upgrade to performance manifolds with through hull exhausts. The aluminum manifolds would save some weight and flow better. any ideas?

For aluminum, I would look at Imco or Gil, Stainless Marine is also good. Not much difference between the 3. For headers I would look at Lighnings. They seem to have the best price of any of them. If the stock ones will fit you application. If not, they will cost a little more, but they can make them fit.

If I can ask, How do the Stock Merc thru hull exhaust compare to these other companies?