If you're looking for 400-450 rwhp, here's what you do. (and trust me, I've been here and done ALL of this myself, and to me, there's no other option)
1. Go to
www.car-part.com to look for junkyard motors in your area. You are looking for a 4.6 PI (performance improved) motor. The number 1 choice would be an aluminum block motor from an 02-05 Explorer or Expedition because they are ~100 pounds lighter. The number 2 choice is a motor from an 01+ Lincoln Towncar, Mercury Grand Marquis, or Ford Crown Vic. I'd avoid police cars. The number 3 choice is a motor from a 99+ F150. You can also get them out of 99-04 Mustang GTs, but they will be twice as much as the other choices here (even though they are EXACTLY the same).
2. Swap the motors. Whatever motor you get, you'll need to swap a few parts over from your current engine. These include: timing cover, oil pan, oil pan pickup tube, exhaust manifolds, valve covers (maybe), and intake system (elbow, throttle body, intake tube, MAF, and air box), and all accessories. If you get a car motor of any sort, you will use the intake manifold that came on that car. If you get a truck or SUV motor, you will need to find a PI intake manifold. Or, if you get a positive displacement blower, you won't need one at all. All of the electronics, wiring, ignition system, etc. will work %100.
3. Put an intercooled supercharger on it. Pick your flavor. There's a billion threads out there detailing the different types and pros/cons of each. If I were doing it all over again, I'd get the Tork Tech Terminator kit and swap on a ported Cobra/Lightning blower, but for someone who hasn't done a lot of work on these cars, that might be a little involved. If I were recommending one to someone who wanted a simple install, I'd say get the entire Tork Tech intercooled kit with their Magnuson blower. You will also need a fuel pump (an SVT Focus pump will be fine for 450 rwhp, and only about ~$100), injectors (03-04 Cobras, or 99-04 Lightnings would be perfect), and a MAF sensor (99-04 90mm Lightning).
4. If you so desire, do a couple of the other bolt-ons. Personally, I'd go with an offroad midpipe of your choosing (if you can get through inspections in your area), a catback of your choosing, a CAI, 75mm TB, and elbow. That's about it, though. Long tubes are nice, but expensive.
5. Get it tuned. You'll probably need an SCT X3 handheld tuner for this, and a decent dyno tuner. This setup will easily make 400 rwhp, and it's just a matter of swapping pulleys if you want more. And trust me, coming from someone who drives a 400 rwhp car on the street, it is a handfull. This setup will be a PLENTY of power on the street for 99%+ of people out there.
If you shop around and get some decent deals on used parts, this entire setup can be done for <$5000. Plus, it will behave completely like it did stock. Very easy to drive in the rain, 25+ mpg, etc. And with the right tires, gears, and driver, it is easily a mid-11 second car at the track.