Agreed LS1Tech is your friend on this subject. BTW Is this for an 96-97 LT1 or 98-02 LS1 car?
If it's for an LS1 car you can go with cheap or expensive headers. Almost all will give similar gains and I have not seen any hard evidence to disprove this.
Cheap Longtubes Flowtech, Pacesetter,
Mid Longtubes Hooker, Jet Hot, Edelbrock
High End SLP, FLP, Kooks, Stainless Works, QTP
I have been running the Jet Hot Lt's for the past two years with no problems with fitment, quality etc. The Jet Hot coating does get a little surface rust on it sometimes but it easily taken back to new finish with some Eagle1 Never-Dull Wadding. As 1FMF mentioned the LS1 cars usually go rich on the air-fuel ratio when longtubes are installed and this invloves losing power normally. However, when the PCM is tuned for the addition of the headers most see a 20-30rwhp and similar rwtq gain making it well worth the money IMO.