Bluenoser,
The 1948 Chevy trucks had the tank under the bed on the passenger side. They are being reproduced, but deffinitely not cheap yet. But the Suburbans and Panels also used the same tank, so if you can find a tank from a 48 Pickup, or a 47-54 Burb or Panel, it will be pretty much a bolt in affair, and the gas gage will actually work like factory!
My Grandfather actually added a 49 tank in my 48, so I have twin tanks in my 48 1/2 ton.
Garry K in Port Orchard, WA