My dad welded lots of gas tanks - not sure he used anything special, I suspect he actually brazed them.
Back about 85 my 72's tank developed a weep in a seam. I pulled the sender and the tank looked mint inside much to my surprise. I bought the alcohol proof tank liner system from JC Whitney and followed direction. The tank is still there today and has never developed the weep again. My Aux tank from a Datsun B210 has developed issues from sitting for 15 years. I bet some sealer could fix it but I think I want a bigger tank than the 12.5 gal. plus it needs a shield plate - rocks!