The tank for your '46 1/2-ton panel is Group #3.001, Part #3692398, and has a capacity of 15 gallons according to the Chevrolet Master Parts Catalog. I emailed you a pic of a NOS one so you can see what it looks like. It hangs from the right frame rail on brackets that are riveted to the frame and is held on by metal band straps. The '47-'48 PU and AD panel gas tank is very similar, but slightly larger at 16 gallons. One of the later tanks might be easier to find and not too hard to adapt, as the '37-'46 1/2-ton panel tank will be tough to find in good shape I'll bet.
