Put self taping screws back into the holes and use some sort of liquid gasket material to seal the screw.

To weld a gas tank you would need to pull the tank out and then vent the fumes out of a tank. It would be cheaper to just get another tank from a junk yard.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200