I hav a 39 GMC long bed and had the same problem. I took a gas tank from a 51 chevy (maybe 52). I removed a section (shorter)so it would fit, welded it back together and used body solder over the welds to ensure it would not leak.
Once cut in half I was able to fabicate a spot for the filler neck and and used the orginal hole for the sending unit. Also, I made a place (like newer cars) which the sock fiter goes in and fed with 5/16 line to the pump.
Before I welded the tank together I cut a 2 inch slice from part of the tank I did not use. I then spot weded it in one half. This was done so the other half slid over like a glove for strength.
After all the welding was done, I tested it with air then sealed it with a sealer.
One thing nice about the tank it had a place for straps. I used the same spots to support the tank.
Hope this helps.
Jim
