Also check the product reviews on Stangnet. They are pretty comprehensive. I just installed Borla Stinger Catbacks on a 2004 Mach 1 based on reviews. Went in very smooth, but grommets are a pain in the butt. Good sound, low rumble at idle, heads turn when you accelerate. No drone at highway speed, particularly in the 1800-2200 range. Very smooth and sharp. Be aware that the tips are 4" rolled, and some may not like the size. (Good home for birds and small animals?) I think 3 1/2''s would be better, but wth. Probably smoother downshift noise with an x-pipe, but I am still using the stock h-pipe. Go to mustangexhaust.com to listen to sound clips of different combinations.
The Bee
2002 S-281 #382 True blue, tan vert
2004 DSG Mach-1