I have this same problem on a 03 silverado 2500hd 6.0
Wondering what came of yours?
P0306 / P0300
We've pulled and replaced spark plugs, wire set, Ignition coil, fuel injector. Good compression 125 psi. Good spark swapped coils multiple times. Checked for scoring on the cylinder walls none, valves are opening and closing. Push rods are good. Grasping at straws at this point.