Well I havent dynoed in a long time and I did some things after I did so I dont know what Im currently at. But if I recall correctly the last numbers I had were 270rwhp and 303rwtq.
I would start with the basics since your in no rush to do anything. I would work from the rear of the car forward. Cat-back, H or X pipe, headers, CAI, Udp's. But thats my preference, there are people that like to do the complete opposite. Keep in mind that none of what Ive mentioned above is going to give you any real power. Basicaly the motor will breath a little better and sound great. But it will all help when you start with the heavier mods that can really take advantage of better flowing exhaust.
The transmission in our cars is fairly weak. So dont go abusing it too much. I went through two Ford units prior to getting the one I have now. (and that was just with hard street driving.) You can read about it here
www.lentechautomatics.com There are ways to get more power out of them if you know how to rebuild them and replace the weak parts. You can also add a shiftkit or valve body to improve shift firmness and shift points, you can even change the torque converter to aid in launching harder. But depending on the amount of miles on the tranny and how much abuse its taken I would prefer to save my money and just buy one that was meant to be abused. Im basing that on my personal experience. Others have had better luck than me in that dept. As far as shifters, thats really a personal preference. Im still using the stock one.
Gearing? I use 3.73's. Some love the 4.10's. If you do a search on this site youll see that the debate is never ending over "which is better". I chose 3.73's because I felt they were the better choice for me. I think they have a good balance of street use and racing.
Here's are links to some helpful threads and subjects weve covered in the past. Its a good place to start.
http://www.allfordmustangs.com/forums/showthread.php?t=8750
http://www.allfordmustangs.com/forums/showthread.php?t=8927
http://www.allfordmustangs.com/forums/showthread.php?t=1337