Well it's very hard to use your stock wiring harness. You can't use your stock gas pedal. Can't use stock ucu without a special program. And there's other things I just can't remember. I had that idea too when I was offered a free 3500 mile 3v out of a 2005 mustang
GT a while back did some research and it's very difficult. Do able but time and money is what's needed.
But if your interested here's a company that makes kits.
2003 GT: JLT CAI, summit 75mm TB, C&L plenum, 24# injectors, Mac Prochamber, SLP LM1 cat back, 3.73 Gears, FR500 Diff, FR500 diff cover, Jmod trans and shift kit, Max Motorsports subframes, Koni oranges front and rear, DBA discs front and rear, shrader performance rear seat delete, APSX D2 wideband, MPT happy customer.