In reality, they are all going to run about the same performance wise. The ones that go into the fender wall aren't too bad...but try and avoid them in case their are future plans for the car just to avoid the extra bend that causes some turbulance.
I would check out the K&N FIPK, I think JLT makes one for the GTs that you can also have color matched to your car, Densencharger makes a good one but I am not sure if they have one for the GT off hand, MAC isn't bad I Guess, I just don't like MAC in general, and Amazon Racing makes one that may fit a GT since it is just the box part and not the intake tube.
One thing to keep in mind, make sure that you get one that comes with a box or something to keep it out of direct access to the engine bay. When you think about it it makes sense. You don't want to be pulling air from the engine bay as it will be hot air...you want it to come from the fender wall.
Also, in reality, you can get about the same kind of gains from just buying a K&N filter for your current air box and removing the air silencer boot that goes into your fender wall. The difference will really only be about 3-5 hp max...I went this route on my 2000 GT I had. Really made the engine sound meaner...but the difference in gains for a NA GT will be minimal either way you decide to go.
