The gas tanks are totally different on the more common pickup versus the panel truck/suburban body, though the two frame rails, drive train, and front sheet metal are the same on each 1939-46 ½ ton.
On pickups the 18 gallon tank sets comfortably and safely within the seat riser and below the seat cushion. Over a million of these pickups were sold during this production period.
The panel truck and suburban were totally different animals. They had no protective seat riser. In fact, there was not even a passenger seat in most panel trucks. For protection, their 16 gallon gas tank was placed inside of the right frame rail and under the body. This gives the tank the safety of the frame rail and being in front of the rear axle. In the attached photos, the totally different shape of the panel and Suburban is quite obvious.
Gas tank removed from a 1941 panel truck.
Under the seat tank (usually on smaller trucks)