I will third these opinions
I have cut them before using other methods( very dangerous) so dont play with fire here...you will get burnt.....
Take it to a radiator shop n have it cleaned and fill it with an inert gas (not hydrogen please) such as nitrogen or argon.
You also didnt mention what material the tank was constructed of Al.,steel or other!
Like I said before Ive done it,but Im not gonna say how as its a "Do not try this at home" deal