yes, universal for them will be better off, then have the shop fab up some nice piping for everything. All of the pre-fab setups for n-bodys are stupid overpriced. I wouldnt run a factory muffler with everything else personally, you'd be loosing a fair amount of flow that you just tried to gain, not sure about how that'd sound either.
http://www.ebay.com/itm/2-25-x-12-OA...9f8ef3&vxp=mtr is the resonator I have. good bang for your buck. Look around for cats, prices can really vary. Check summit racing and ebay of course. If you want to pay for the high flow, go for it, some say its not worth it. Flowmaster also makes good stuff but I think the magnaflow's sound better with our motor. Do some checking on what you want sound wise and are wanting to pay for your muffler, lot of options from even just those two brands. Keep in mind you want to keep some back pressure. If they pinch bend pipe then I'd go 2.5in (in the 'pinched' bends it could narrow down to 2.25in) if they have a bender that doesnt crunch it 'Roll bender?' then you'd be fine going with 2.25in for high flow, but not loss of back pressure.