ZTune up,Intake(CAI) headers,and 2.25 exhausht with a 2.25 glass pack at the stock local.It'll sound great with a lot of burbling when you let of the gas and cost.Change the rear gears to 4.3 if there not already,that'll help keep the rpm down as well and going to a size bigger tire.
The chevy muffler would be a good idea,but I think its just to big,there what 2.75 or 3 inchs,way to much for even a built 3.0,it would kill the torqe.And there big,there a good 30 inchs long and heavy.A good glasspack is still cheaper and would probly last longer since its not as big.But if glass packs are illegal in your state then you should look at a decent turbo style muffler,like a thrush or cherrybomb.