I recently had some good advice about upgrades to my 03 gt, I installed 3.73 rear end gears. Also had a set of sub-frame connectors and of course air intake kit. Also bought a diablo tuner. Wow what a difference. But now I am asking what if I bought a chip for the car and use the tuner for it will it work. and aslo about wheels. Will 20inch wheels fit if I go and install an eibach springs set 1.5 drop all the way around. Or stick to buying 18inch. I also want to install headers which is the better make. I was told buy a performance shop that mac is the way to go. Can anyone help.