I don't know if this will help but a decade ago had a 2000 mustang gt and modified her a little.
on the exhaust i did the bbk shorty headers, JBA very good! get new heavy duty exhaust gaskets. and there is a thead on here about high quality 8-point header bolts you will want those, you dont want the oems they can back themselves out. it was easy for the stang i got the whole kit from american racing, it was magnaflow
with the pieces from the headers to the tips. a option was 2 high flow/performance cats (warrantied) took a long weekend and did it myself, and they never failed 3 Chicago emission test. had 2 assholes who turned and looked when i pulled in, they did all they could to fail me but hey I was legal and my emissions were lower than 2 years earlier, point of this, is to try to keep the cat on, it can become one long headache. i went form 4 to 2 with a true x-pipe 3"from start to end exhaust
also put new o2 sensors while i was there.
one thing, the shorty headers get hot, man I mean hot, like bake for feet hot. its not as tight as with the stang so you will get some cooling, but I wished I had wrapped mine.
now if you are not 100% sure you can do this, dont, its a job and one mistake will ruin it.
But the end results are so sweet!