custom exhaust/muffler specs?

So I'm in the process of slowly making a custom exhaust for my 08 CTS DI RWD. I'm starting off small by replacing the mufflers with Borlas XR-1's, and I was wondering what the specs and mounting style others have used are. Should I get offset in, center out/ center, center, etc? And to those that have done this, did you install where the factory mufflers were, or move them closer to the resonator? Thanks for any help.