I have an LS with an LS vtec in it. I have a cat back for an LS and need to change the cat and header. Do I go with LS cat and GSR header or what deadly combo do I need. I have been told that the LS flanges differ from a GSR flange or does it not make a difference???

 

The flanges are identical.

The A-pipe length and cat location differ between the two. An LS header and cat will bolt right on to your current exhaust.