Alright, I started this whole mustang mess when I was 15 so I know where you are coming from. After the intake and carb, I would go for headers and duals. I put on a factory style GT system. Sounds pretty good, and is rather hush hush for the cops (good idea at 16). I am 27 now and am looking to put flowmasters on with a 2 1/2 inch system all the way back. Louder, but I can get away with it now. I would say the next place to go would be rear gears. You probably have 3.00 or 2.something in there. Jump to a 3.55 (don't go higher for a street car you will be sorry). Get good gears, richmond or similar. Summit brand is not the place to save money. I upgraded to a 9" rearend cause I had one sitting here. But from what I understand unless you are going to be dumping 350 hp you just don't need to spend the weight (9" are heavier than 8") or the money. Also note that while doing this stuff seriously consider improving handling. Disc brake conversion up front, bigger front swaybar and addtion of rear one ($200 project), good shocks, quicker rate leaf and coil springs, ect. As my daddy always said, "you need to be able to get out of trouble as fast as you can get into it". Feel free to email me, I will be happy to give you as much info on my experiences as I can and good luck!
CMM
67 mustang coupe
70 mustang mach 1
88 mazda/ford mx-6