I'm not sure what the Stage I's have. But the Stage II's have a better suspension, body kit, side exhaust, Fancy chrome rims, better tires, and some floor matts and stuff. They are sweet looking cars, but you'll pay for it.
The Stage III's are the real performance beast, They have a Supercharged 4.6 SOHC with 360 HP (I think?), better suspension, and all kinds of stuff. They are very similar to a Saleen S281 SC. And as already pointed out, are just as pricey.
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1995 Mustang GT