[redmt]

The problem you may find is that not many fab shops will weld on a gas tank. If I was to do it, I would first flush with plain water then add some dishwasher detergent ( that stuff cuts anything) Fill the tank with a gallon or so of water and shake the snot out of it. Rinse and repeat. Let it sun dry and then see if you have any hint of fuel smell. If so do it again. You would have to purge the tank of any fumes to weld it no matter how you do it.
If your going to try and mig weld it I would look into a dual shield wire and then still flow purge gas into the tank while welding.
On a guess by the time you buy wire and a stainless patch piece it would be cheaper to find someone to weld it at a shop. You will still have to have it super clean before they would even touch it. Be prepared to pay $5.00 or more per inch of weld. I would have a $50.00 minimum for just your reference.