You absolutely MUST consult your owner's manual. This is crucial because:
* Different Transmissions: Ford Econolines used various transmissions (e.g., different versions of the TorqShift automatic) over the years. The fill location can vary significantly between them. Some might have a dipstick with a fill tube, others a separate fill plug.
* Location Changes: Even within a single transmission type, the location might vary slightly year to year.
Attempting to find it without the manual risks adding the wrong fluid, overfilling, or damaging the transmission. The owner's manual will have diagrams and clear instructions.
If you don't have the owner's manual, you can often find a PDF version online by searching "[your year] Ford Econoline 350 Super Duty Owner's Manual" on Google or the Ford website.