Welcome to the Mustang community!
TBone has a good list for what you're looking for....
As far as the underdrive pullies go, I live in Dallas using the Ford Motorsport pullies.
We've been over a hundred every day for the past three months (105 today), and I've had no problems. You will notice the battery re-charge guage get a bit low if you sit in stop and go traffic for a while, but that's about it.
Non-Dyno tuned chips are a bad idea in my opinion, and hardly ever deliver the numbers they quote on our modular engines.
3.73 gears and a good shifter go hand in hand. You can practically kick the Steeda tri-ax from 2nd to 3rd...
The easiest way to think of the X versus H pipe debat is this:
Generally, an H pipe will deliver a bit more low end torque than an X pipe, while the X pipe will deliver a bit more HP in the high end. It's all about your style of driving...
In my opinion, An after-market cat-back system on 99-00 GTs are waste of cash unless you're looking for a different sound. The stock cat-backs flow quite well for near stock applications...
Hope this helps...
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"The Yellowjacket"
1998 Mustang GT
13.97@103
"Traction Impaired" or "Nitto Needy"
"They think they can make fuel from horse manure.... Now, I don't know if your car will be able to get 30 miles to the gallon, but it's sure gonna put a stop to siphoning."
- Billie Holliday