Yikes, that is an open-ended question. Where do we begin? Your gears are 3.23. Many GT guys go with 3.73 or 4.10's. I personally believe that the 3.73 are the best match with the GT and the 4.10's with the cobra, but you know what they say about opinions.
Your fuel injectors are 19 lb...they can handle much more HP than stock. You generally don't need to upgrade these unless you are adding a blower or something like that.
Best thing to do it keep checking this site. Look at the mods some of us have done and feel free to ask any questions. My brother drives a Z-28 and you will notice that Mustang motors lend themselves to different mods than the Camaro.
Some of the most popular mods (and I don't believe I am really addressing your question) are:
- Pullies
- Gears
- Exhaust
- Intake (like cold air) and plenums
- Power chips
- TB's
- Shifters
Your motor is rated at 260 HP at the flywheel and around 230 rwhp.
Some popular parts sites and my favorites:
www.mustangtuning.com (for the easy stuff)
www.sacrementomustang.com
www.crossroadzdirect.com
www.cjponyparts.com
All of the above listed are good for late model cars. I know I left out a bunch, but I am sure the other guys will help.
Also, start reading Muscle Mustang & Fast Fords. Lots of great info on where to find parts.
Hope this helps and welcome to the dark side.