Does this fit a 99 Mustang GT?

 

if your an auto then yes

 

Why would you need one? With a tuner you can firm up shift points, change the RPM's where it will actualy shift, etc. Installing a shift kit is more for a car that cant be adjusted with a tuner IMO.. I looked at them when I had my 6er.
EDIT: one exception is if your REALLY beefing up your drive line... but I would do everything in my rear first...

 

That's not true, a tuner will firm up your shifts but not as much as a shift kit that will give constant line pressure. With just a tuner u will still have to build pressure to shift, it won't be immediate like it wil be with the kit.

 

IMO shift kit + tuner = best way to go!!! I have a Trans-Go shift kit and have adjusted my like pressure with my Predator and love the way it shifts!!!!! I have had this set-up for a little over a year and have not had any problems at all.