I have 2 custom made stainless steel back boxes which bolt straight on to a 2006 6.0 Monaro VXR but are large oval pipes, gives a nice sound. PM if your interested.
You'll have to be sure you get something suitable for the VZ 5.7. I don't think the Yanks got the 2005 5.7, so don't expect a Pontiac version. Twin, twins will probably foul the bumper.