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I just had Cartek in NJ install their 2X H&C package in my '02 Z06, along with their HD clutch system ,ported intake manifold, TB, Double roller timing chain/gear, PCM tuning and several other stenghing mods with headers and stock cats intacts.
The end result is 439/407.
I'm sure they will sell you the the parts and assist your local tuner for proper install and advice on PCM tuning.
Cartek is a supporting vendor of this site...you can find their banner and click onto their website on the right margin on this page.
Ask for Dave or Julio...they will share with you in detail what exactly was done on my car and others.
I'm sure there are several other great tuners such as LG (supporting here as well...Lou is a top notch tuner and a great guy...you can click onto his site in the right margin as well) that offer great packages as well.
Do your DD, ask alot of questions then decide....make it a fun and interesting experiece so you know what to expect... :)
I've been following this same issue for a couple of years. My shop (Tony's Corvette Shop) is very very conservative about their suppliers. After much research they recommended Cartek. Tony's installed the Cartek Stage II package on my 02 in February. You can see the numbers in my signature. There's 5K miles on the engine since the mods without a burp. The car still gets 28+ mpg on the road. Cartek has a bunch of H/C packages on the street and no other tuner is producing more horsepower - their customer satisfaction is also very high. To do it right you'll need more than heads and a cam. To achieve the full benefits you need long tube headers, an air intake system, bigger injectors, a few other minor goodies and a custom program in your PCM. With all that you can count on between 430 - 460+ rwhp depending on the package you select. The last time I checked Cartek Stage IIX packages had to be done in house. Call them and ask for Dave.