I have a new set of MBRP T304 stainless steel axlebacks with 4.5" slant-cut double-wall tips. Here's link to them on CJ Pony Parts' website:
http://www.cjponyparts.com/mbrp-axle-back-exhaust-system-3-pro-series-gt500-2011-2012/p/CBP7/
The muffler cases do have a couple dings and marks from shipping, but otherwise these have never even been installed. The instructions actually say they fit 2011, 2012, and 2013 GT 500s, but I'm not sure how they'd look in place of the '13 quad tips. See pictures below. Asking $425 plus shipping
http://www.cjponyparts.com/mbrp-axle-back-exhaust-system-3-pro-series-gt500-2011-2012/p/CBP7/
The muffler cases do have a couple dings and marks from shipping, but otherwise these have never even been installed. The instructions actually say they fit 2011, 2012, and 2013 GT 500s, but I'm not sure how they'd look in place of the '13 quad tips. See pictures below. Asking $425 plus shipping