What's the best way to go about modifying my 2005 mustang gt?

I'm only looking for about 420 in horsepower

Go to americanmuscle.com and research cold air intake/BAMA tuner combo kits, look into getting a ford racing high flow intake manifold, and larger throttle body as well as a ford racing exhaust. Be sure to mention when you buy the tuner each and every one of these mods so they can give you the best custom tune they can. Expect to spend $2,000-$2,500 for parts and get about 50 horsepower. I also recommend 3.73 gears. Keep in mind this will require very skilled work, and if you need ford to install these items it will cost hundreds of dollars.

Getting to 420 will require extensive work (high flow heads) and/or forced induction.

Go on Jegs. They have all kinds of bolt-on mods such as a bigger bore throttle bodies, performance intake manifolds, performance camshafts, supercharger kits, even little things like slightly cooler thermostats so that it injects more fuel to the cylinders. There's literally endless supply of performance parts for the Mustang.

There are more bolt on go fast goodies for the mustang gt than any car ever produced. It all depends on what you want to spend. Cold air kits arent really gonna do much without alot of other mods to go with it. Exhaust mods help out some. The 4.6 is on the more expensive side to modify i miss the old pushrod motors

You're only looking for about 420 horsepower? No problem- just change the fuel injection, intake, heads, pistons, rods, crankshaft, cam, lifters, ignition, etc. Then, the first time you hit the gas good and hard, it destroys the stock tranny and rear end as the driveshaft is coming through the floorboard.

Leave it as is. Unless you are planning to win races on the track.