Are you going for the classic "raked" look? The Mach's are 1/2" lower front and back compared to the GT's, so often the heights they give for springs on the websites are drops for GT's.
Air shocks would make it easy to raise the rear for sure, but it might effect the handling quite a bit. I'm not sure what would be the best for raising the rear.
For the front, you might take a look at some of the different spring heights and you might find a set that only lowers it 1/2" to 3/4".
Sent from my iPhone 5s with iOS 8.0, but I can't really tell the difference, so it could be iOS 7.0 :dunno: