The tank would weigh less than the truck. We ran bob tail delivery trucks just like you have pictured in the 50's. Before you do any work on it, take it to someone who works on gasoline tanks and have it degassed. Vapors can hang around a long time, and it's the vapors, not the liquid that explodes. Kraft Tank in Kansas City deals in tankers. They can either do it or refer you to someone who does. If it was a tanker going back in service, it could be filled with an inert gas like nitrogen. That's what we do when we weld on them, etc. However, you probably never intend on hauling gas, so it would be better to have it degassed.
