i bought a speedcal from dallas mustang when i put 4.10s in my 96. worked fine and 2 years later it took a crap..... but it coulda been a fluke, worked how it should when it did work.

 

I had a speedcal on my 2000 mustang and it worked fine.
However, I would suggest getting a SCT Tuner if you can afford it. In the long run it will be a far better investment since it will allow you to alter tunes for specific mods, etc.

I would perform the activity described above to determin what gear ratio you have(unless you have a GPS).