The red car I posted above looks to have glass packs which I'm not going to use, I currently have those and their a little noisy...I'd like to tone it down a bit also. I'm looking at Flowmaster mufflers, single inlet 2.5" or 3" with 2.5" dual outlets. These come anywhere from 14-16" long with 10" wide bodies and 5" thickness. They should fit nicely under the rear body work. Painted black with two nice chrome tips, they should perform well and look awesome.
If you want something quiter get a longer muffler. Flowmaster has this in a lot of different dimensions. I've seen chrome tips with an angled cut with about a 3.5-4" diameter (just saw these on an Infiniti Coupe this morning). I think this should work and look pretty good. Here's a link to Jegs with a muffler that has the above dimension and should work.
http://www.jegs.com/i/Flowmast...fMK5B8CogMLB2zzw_wcB