Not in your kneck of the woods so much, but I'm in Snellville and have several places nearby that will do it without question.
Andy's Muffler (they have done excellent work for me and a guy who's good with custom Mustang exhaust)
Gunter Performance (Lawrenceville): Wayne Gunter's shop will put anything on your car without question and will warranty his work. He specializes in Mustangs of any year and I believe he was the head mechanic for Dugan Racing for years. Has his own shop now (Dugans closed for about 9 months a while back). Quality work and no hassle getting anything done.
If you're looking at subs like Maximum Motorsports full length ones, I'd have Gunter do it for me. There's got to be a shop like that in your area. Definitely post in the Southeast section, as that is how I found Gunter.