I agree with emptying the tank for safety's sake. But the vapor (read: combustible fumes) space in a fuel tank, whether it's empty or partially filled with diesel, is what explodes and causes damage. In my experience from being raised on a farm, you should empty the tank of all fuel and fill the tank completely with water to eliminate the vapor space. This being said, again, I know a guy who used to weld cracked natural gas lines while the lines were carrying natural gas. I would not like that job! Once oxygen gets in the line, or tank in this case in the vapor space, then you have two of the three components for a fire and explosion. Add enough heat from welding and kaboom! Oxygen, fuel and heat...