What I'm saying is that unless you haven't got a budget to worry about, that you're likely not going to meet your goal.
Either that or you'll end up taking a short cut and grenading the whole motor.
Were it me... I think I'd buy the car that I had my heart set on (in this case a GT). Go out and do some research on supercharger systems. Pick one out that is SAFE for the stock motor but also allows you some room to grow (through increased boost).
As you're able, you can build up the motor, drive train, and
suspension components as you go.
Off the top of my head, the newer 2.6L, liquid cooled, Kenne Bell will dish out more boost than the stock motor can handle. The 3.6L puts out even more (although I'm not sure it's a available for the 5.0).
At any rate... Take a look here:
http://www.kennebell.net/KBWebsite/SC_pg/Mustang 5.0 (2011)/layouts/Mustang2011.htm
After you've read through everything, call them up and ask them questions or send an email. Find out what a complete kit will cost you including throttle body and supporting mods. Tally it all up and see if something like that is an option for you.