The srt8 headers cannot fit on the 4.7V8. It all depends on how loud you want and how much drone your willing to deal with. Usually the louder the muffler the more drone you will get on the freeway. Running true duals is very tricky. There isn't a whole lot of room for the dual mufflers. You should think about a dual inlet/outlet muffler.
__________________ 2010 Brilliant Black SRT8: MODS = To much to list! Check out My Build
I just installed my Gibson Performance slant cut exhaust cat back. I absolutely love the sound. Its the best of both worlds it sounds great but isn't too annoying loud. low rpm or cruising you can barely here it till you start getting into the throttle.