well, for the gtx part, they don't make pontiacs anymore at all, so that's a no lol. with headers, you don't need to get a downpipe. if you get headers you will be replacing the downpipe part of the exhaust as well with the same piece. if you don't want headers, you will need to get a downpipe and what's called a powerlog, or plog for short. it's basically a replacement front exhaust manifold, just fabricated for better flow. i haven't done it, so i'm not sure on the skill level. hopefully someone else will chime in on it. getting it scanned for KR should be a priority when you get some free time, until then stay out of wide open throttle or other kinds of hard driving.