Don't want a pieced together shop made exhaust. Mandrel kit is better even with some modification. From what I've seen on people mod list they run duramax aftermarket setups just want to verify before I order one.
A custom built or go to Magnaflow they have a kit for the 2500, not 4 inch.
For the 6.0 you only need a 3 inch system. A 4 inch system on that engine will drone like crazy.
I had a 8.1 Avalanche and i used the stock system back (dual cats 2.5 inch) to Flowmaster dual in and dual out Big Block muffler- deep tone and a little drone- and dual outlets out behind each rear wheel.