The lighting is a 5.4L modular motor. the heads won't fit on a 4.6L. notoriouspig was right about what PI heads are, but if it's from a lightning it won't work for you. Since you already have the PI heads on your stock 2000gt, the only way to make the flow better is either 1. port and polish your heads, 2. get an aftermarket set of them, or 3. add an air induction(i.e. turbo or supercharge it) Or you could get different cams like crower, eddelbrock, lunati... etc...etc... there is alot to take in when it comes to modifying your car, and can be expensive.