I am one of the people that say the Mustangs suspension and chasis are barley adequate for the stock power levels. If it were me what I would do, and have done several times, is work on those areas first.
1. Subframes. One of the single greatest mods on a Mustang.
2. Suspension. The complete Eibach Pro kit with the springs, struts, shocks and sway bars is a great inexpensive complete kit that will make the car sit nicer, hook better, ride better and corner better. Just one example as there are thousands of ways to do a Stang suspension based on budget and prefrence.
3. Brakes. Did an EBC rotor and pad upgrade on my friends 2004 GT (he did it I should say) and took it to Harris Hill (look it up) and proceeded to beat the hell out of the car and he said he has never been so impressed with a simple upgrade like that in his life. Said car had less fade then the 2000 Cobra R he used to race. This guy mods GT500's, KR's, SUper Snakes, Teralingua, Ford GT Supercars etc etc so if he was impressed then ya know its a worthwhle upgrade.
4. Camber Caster plates as the car will need them and also a bumpsteer kit.
Now the car is ready for some power!
5. Off Road H-pipe. My favorite because it will give you a deep tone and substantial HP gain. A X-pipe will give maybe 1-2 more peak HP but sound raspy IMHO. Pro-Chambe has the best tone and has the power of a X-pipe but they are costly
6. GEARS!! Personally I like 3.73 but if you dont do a crap ton of highway driving then 4.10 is ok.
7. Plenum elbow. They are cheap and really work. I have seen only 2 dyno runs with them but both cases picked up 8hp and 11hp.
8. Tune. Real tune. Yeah it costs more but all can tunes will either leave power on the table or run the car dangerously lean.
Headers and cams do provide a power increase but to me they are too expensive for the gains offered. If I bought another 2v Stang then I would do TF heads and perform the header and cam upgrade at that time to save on labor.
Hope it helps.
