Best sounding is subjective. Honestly if you still have cats and can't stand the loudness at 4k with those mufflers you might not want to modify it any further lol.
The Youtubes has a ton of exhaust vids you can browse through for starters.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.