If you're worried about it drop the tank first. You could pack the top of the tank with sopping wet rags too, but when they dry out they'll catch fire which is even worse.
I'd probably drop the tank.
Always a good idea to have a good extinguisher handy. Make sure it'll put out a gasoline fire (water will make a gasoline fire worse)
Prolly a good idea to have someone keep watch while you're welding too. You will not notice a fire while you're welding...
Rene