I have had my 2013 2500HD Duramax leveled by adjusting the torsion bars to fit 285/60R 20 tires. Truck now rides very stiff. I believe I need a longer shock but for which ones is my question. Do I go with 0-1" lift or more?
^^. Your factory shocks will not limit stroke though the travel because you haven't changed any parts. You're likely so cranked up that you have no "sag".