There are quite a few headers available. There is obviously less room with long tubes than short tubes, but both will fit. There is very little to gain from short tubes though. JBA and Kooks are two off the top of my head that make long tubes, but there are several others.
If you do longtubes which I would if I were you, you will also need a catted or off road mid pipe. JBA makes a nice h-pipe with high flow cats. I'm sure there's at least 5 different brands of headers out there. JBA, Kooks, BBK, Hooker, Bassani, etc.
Longtubes are definately a good modification, but if you are looking at hp per dollar then there are other mods to look at first. The stock manifolds are not overly restrictive, especially if you do not have forced induction. The cost of the installation can cost more than the headers themselves in some cases.
JBA headers are a good mid-range header, not as expensive but not as good as Kooks and ARH. Unless you really want every single HP and are willing to pay for it I would choose JBA.
Also, a good ceramic coating is a nice touch to keep underhood temps down.
Dan
I have the ARHeaders and they fit is great. I also know that Tillman Speed is having a great Christmas sale on Kooks which are just as good. Give chris a call.
I have Kook's long tubes on my automatic '08 GT and they fit very nice. Install was done on a lift and we un-bolted the lowest motor mount bolts so we could gain an extra half inch of clearance for install. Other than that, the went in very well. Took two of us about 5 hours. Seat of the pants feel is fantastic and the dyno showed 18.5 rwhp across the board and almost a 27 ft/lb increase, even at low rpm.
Well worth the money, but I second peoples comments about shorties not being worth much. They do change the sound a bit and make the car slightly louder, but only a couple horsepower increase.