To be absolutely certain, you should:
* Consult a repair manual: A Haynes or Chilton manual specific to your 1998 Chevy 350 truck will have diagrams showing the exact location.
* Use a diagram: Search online for a "1998 Chevy 350 engine diagram" to find a picture that shows the sending unit's location.
* Look closely at the engine: Once you have a visual reference, carefully inspect the engine block near the oil filter. The sending unit is relatively easy to spot.
Be careful when working around the engine as it may still be hot. Allow it to cool completely before attempting to locate or service the unit.