I have what I belive is a 1949 or less. Maybe a 1948. The tank is under the bed and is pretty standard in shape. It seems like any square tank would work if you can strap it up there.
If you can build out of aluminum, that is the way to go. You can cut the pices with a standard skill saw and weld the edges. Here in Florida, there are lots of airboat places that build tank pretty cheap maybe under $200 for an aluminum tank.
I built one for my car (yes car not truck) still a gas tank. You can see a pic at:
Its also a good read on what not to do! I made a few mistakes. But saved money and built a cool tank.
You could always use an old beer keg, I am sure it will fit and if you get a keg you can drink the beer and keep the keg. You may loose the deposit, still cheaper than a tank and more fun than welding it.