For those that are concerned about your original behind the seat gas tank, there is a solution. More important, it does NOT require placing a flat tank under the bed behind the rear axle, fuel filling must be through a new hole in the bed wood, and having no place to place your spare tire. It is filled through the right bed side exactly like the beginning of the Advance Design years 1947-48 (actually most all GM trucks before mid-1955 were filled in the right side of the vehicle).
A complete change over kit is now available from Jim Carter Truck Parts as part # MEG159. It is an exact copy of the 1947-48 ½ ton design. Of course, on your 1949-55 ½ ton you will need to cut an approx. 2 ¾” the round hole in the bed side for the fill pipe.
The following is a photo of the original 1947-48 round fill hole and the dimensions to help you make the positioning just right.