Can't go wrong with a procharger

 

Now just my 5 cents here.
Not sure on the turbo gains but a super charger will put you in the range of a tuned 4.6 around 325 or so. Will the bottom end support boost? It will for a time. When it lets go you will be looking at a good bill to build a forged bottom end for your V6.
What about the rear end? Well you will have an 8.8 I believe but no posi trac and some big gears at least 331s. You will want to get some 410s or 373 (i would go to 410 with this HP) and a posi.
What about stopping it?? Brake upgrade.
Now suspension. You will have wheel hop now have to deal with, so upper and lower control arms. You will want to get rid of the 4X4 look Springs, bumpstear, Shocks and struts (if there stock from 99 they are toast and were not that good to start with).
Rims and Tires you are not going to hook on stock tires

You will have a car that will surprise the hell out of a stock mustang GT.

Mind you I have spent close to 10K of your hard earned money.
I would just enjoy the 99 Stock and wait to put that sort of coin into a GT. Your car and 10k will put you into good GT. A good GT and 10k will be one badass car.

again just my 5 cents

 

Amen to that, and hey it only benefits the mustang name to power up a six and show what we can do to earn our stripes, or ponies so to speak