To be certain, you'll need to consult a repair manual specific to your vehicle's year and engine. A diagram will show the exact location. You can usually find these manuals online (often as PDFs) or at auto parts stores. Searching for "1998 Ford E250 4.2L repair manual" should yield results.
The IAC valve is usually a small, cylindrical component with electrical connectors attached. It might be hard to spot if you're not familiar with the engine bay. Having a repair manual with diagrams will significantly help you locate it.