The 2004 Chevy 2500HD 6.0L doesn't have a readily accessible, manually operated fuel shutoff valve like some older vehicles. The fuel system is controlled electronically. To stop fuel flow, you'd need to either disconnect the fuel pump relay or, in an emergency, remove the fuel pump fuse.
However, this should only be done by someone familiar with automotive electrical systems as improper handling can damage components or create hazardous conditions.
There's no external valve to easily turn on or off the fuel supply.
