Ahemmmm... This from the official GM "Shop Manual"
he "C" code vs. "J" code designation is the eighth digit in the in the vehicle identification number...
For you non-doesel types...the 5.7 L gas motor will have code "L", "M" or "P" depending on variations of carb etc. The 7.4 liter (454) gas motor is a "W".
If the motor is already out of the truck...see previous post...