A-HEM... Ill take this one
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Whats up man, for 99+ stangs you will need one of two things, a Speedcal, or a chip. I went with the chip route with my 3.73's and I must say it took what would have been an average kick in the pants with a gear swap, to a full boot up the arse since I got my timing advanced as well.
The entire car just seems to feel smoother and stronger since I went with a chip.
If you do go with a chip, I would definately recommend Diablo... its what I have as well
. Most speedshops that will sell you a Diablo chip will have the software to tune it for you. This includes a/f, timing, gear calibrations ect... To get the most out of it, you might want to do a full dyno tune, I didnt, but thats your call.
All in all I am very happy with my decision Ill have to say. Price was only around $150 more to get the chip and a custom burn at my speedshop.
Best of luck man!