Welcome Maui!
If you've ever seen a Summit or Jegs catalog, or similar, you know that Mustangs pretty much own the aftermarket for performance parts. That is a good thing!
The first thing I changed on my '88 was the mufflers. I went for the Flowmaster American Thunder Catback system (two chambers). I was VERY impressed with the sound afterwards. After that, I went for the Steeda Unequal length 1 5/8" headers. Made it a bit louder, but the real difference came from the BBK off-road H-pipes. After I installed those, I swore I'd never go back! I love just starting it, and listening to it idle.
You will too, you'll see! When I installed them Florida still had emissions laws, so every year I'd have to put the stock h-pipe back on for emissions. Now that it's gone I get to live with the nice rumble all year long!
Good luck, I know whatever you decide, you'll love! The stock system is good, but the aftermarket has it all down pat! Let us know what you decide!