I don't believe anyone makes an exhaust for the SRT-6. Being that it is different from the reg Crossfire and the SLK cars.
That's the reason I haven't changed mine yet. If and when I do, I'm just going to remove the stock resonator and do a custom Y-pipe after the cats and use the stock muffler at the rear.
I've read of a couple SLK cars and one SRT-6 that have done this and like it.