Where is the oil sending unit on a Chevy 3500 7.4 engine?

The oil pressure sending unit on a Chevy 3500 with a 7.4L engine (a Vortec 454) is located on the driver's side of the engine block, near the oil filter. It's usually a smaller sensor, often screwed into the block itself. It's not always easy to see without some careful looking, as it can be partially obscured by other components. You'll likely need to consult a repair manual or a diagram specific to your year and model of truck to pinpoint the exact location. A picture of your engine bay would help for more precise guidance.