[Troyota]

^ The O2 sensor mounts in the exhaust usually in the header. The moddification needed on an aftermarket header is usually to extend the wires of the oxygen sensor to reach the bung on the header. Aftermarket headers usually have long individual runners for each cylinder and come together further along the header than your stock manifold (which pretty much just comes out and together imediately). That's the only real modification needed to make a decent aftermarket header work w/ your car. Unless your state are real nazi's (i.e. California) I don't see why having an aftermarket header would imediately fail you on an emissions test.

But what do I know...I live in New Mexico (No emission controls)