For an 04 car make sure you get one from a 99 - 04 GT or Mach1. The 99 - 04 8.8 is a little bit wider then the fox and SN95 8.8 rears. They will fit your car better.

Also,.. I've seen a couple really good turbo set ups on the V6 cars. One guy is making 450 at the wheels. So you do have other options,.. because swapping in a V8 will be pretty involved.

Good luck!

Hurst