Clean the crack up and expose clean metal by sanding it or a small grinder. Use JB weld it is a two part mixture that set's up hard as steel. Any hardware store should carry it.
I have fixed gas tanks,glasses and a few other thing's with it.
The main thing the surface has to be clean so the compound get's a grip to the metal. Since you already have a crack the compound will go into crack to get a inside bond.
You may use a small file or sand paper to open up the crack a little by adding a bevel to groove to expose shinning metal to get acurate bond.
Drain tank and let it dry out before attemping to fix.

They make all kind's of JB weld for different types of applications and Lowes or Home Depot will carry it.

Check out this link http://jbweld.net/index.php