header installation question
only real "modifications" that are needed may be with the AIR or EGR equipment, depending on if the headers came equipped with fittings. next thing would be the Ypipe. stock surely won't fit. so you'll either have to take a chance with an aftermarket one, have one custom built...or better yet go TDs
like teke184 said, if they aren't "race" headers then you will need block off plates for your egr/air(assuming your car is a 98-00) if it is a 01-02, then you would just need air plates 
