1949 and up trucks are configured like this for inside cab fuel tanks. The earlier cab has a full length sheet metal liner that extends from the belt line to the floor filling much of the space for the tank. This liner also provides support for the early cab mount center shackle.
http://www.flickr.com/photos/advance-design_parts_co/8130557498/in/set-72157631870734114
The exterior under bed tank for 47-48 (mounts are different) Here is a photo of the 47' only mounts. The next photo shows the 48' mounting both filler necks come through the bedside on pick-ups.
http://www.flickr.com/photos/advance-design_parts_co/11932606535/in/set-72157631868452754
http://www.flickr.com/photos/advance-design_parts_co/8130517575/in/set-72157631870734114
I have never seen a truck built in 1949 (GMC or Chevrolet) or later with this type of tank. Let me clarify I am talking about pick-ups only here.

1950 Chevrolet 3/4-Ton 3600
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