I am going through this right now and I would suggest also looking for aftermarket exhaust.

Are you doing the work yourself?

As for the parts you mentinon you may only be looking at $1000/engine

As for a website to order the parts I would suggest JEGS.com but before you get that far youll need to know your part numbers and I am not that knowlegeable.

What block do you have? gen V or VI

my best guess for carb and intake would be edelbrock performer air gap and holley 800

With the cam you have to be careful about not being too agressive to avoid water reversion.

How many hours are currently on your engines? If you're looking to upgrade your Merc 454 Mags WITHOUT refreshing/rebuilding the bottom ends, then it will greatly depend on how many hours you currently have on your engines.

Also, I am not sure how you are thinking about goint about doing this because if you plan on adding an upgraded cam, then in order to do it correctly the FIRST TIME you MAY have to remove the oil pan to avoid future problems or oil leaks. This means removing the entire engines from the boat. Perhaps someone can chime in about having to remove the oil pan. If you are not willing to remove the oil pans in order to install the cam, then you should skip the upgraded cam idea.

The other thing is if you put a cam or more power to an engine that is already too "tired" you may experience more blow-by or even more excelerated wear on the rings/bearings, etc, and can bring even more problems. There is much to consider.

The only way to get close to 475hp with stock heads is going to be with a roller cam valve train. You're looking at about $2500 US for parts for a roller cam, lifters, timing set, springs, gaskets, valve job, intake and carb. You will need to upgrade the stock exhaust to work with any cam upgrade at about $2000 per engine as well.

Being its a carbed engine, how many hours are on the bottom ends????? If its more than 400hrs, I would consider total rebuilds.

Hi guys thanks for the input engines have done a total of 120hrs so dont think they are tired. yes am aware of engines having to be lifted from the boat I feel that my bottom ends are in great condition.

is a 800 cfm carb , header style manifolds , cam and roller ancilliarys enough to chase the high 400hp mark?

You should make 450-475hp with those upgrades. With only 120hrs, I wouldn't touch the bottom ends either. With a new set of heads, you could probably make 500+hp.

what sort of money are performance heads for my 454 mag its a 1995 engine

If you only have 120 hours on the engines, then you are good to go for the upgrades. You really don't need upraded heads for 475hp on a 454cid engine. If it is the Merc 365hp Mag engine, then it already has the GM rectangle port heads---just have them lightly bowl/pocket ported, 750 Holley carb is plenty, aftermarket manifold style exhaust system, and a hydraulic roller cam/lifters will be good,

You might want to contact "RMBuilder"/Bob Madara of Marine Kinetics in N.Y. 585-654-8583 for your cam application. He is the expert in marine cams and has helped several guys like you here on OSO. He did my cams 4 years ago. He is extremely knowledgable and is a great help.

Probably looking at $2000-2600/pair.

Like kaama said, you could get some custom porting done to your heads also. I don't know if you have anybody there taht could do this and shipping to US would be high $$$. For a valve job, springs and port work you're probably looking at close to $800-1000/pair.

Like Kaama said, contact RMbuilder. He can set you up with a package.

So on the conservative side does 120hp additional = another 10 mph with the right prop setup?

I'd say thats about right depending on your hull, drive and prop efficiency. Make sure you upgrade the springs on the heads to match your final cam choice! Good Luck, have fun!
The people reffered and mentioned here on OSO can keep you on the right track and budget.

Best Regards,
Ray @ Raylar