Fuel tank repair/restoration

A picture would be helpful.

I had exactly the same failure earlier on this year. Considered weld repairing it, but didn't want to knacker the paintwork so i counterbored the two mountings out. That left two 20mm holes, then turned up two screwed plugs that used a thin nut to seal with a copper ring on the inside, nut on the outside, had to feed the plug & copper washer through in filler cap hole, wangle into place then screw the nut on and tighten. I had already tapped the plug 5/16" UNF for the mounting studs. 2k miles so far and still sealed. Whatever you do, when remounting the tank, measure the gap between the tank and bracket and use enough rubber washers to enable the tank to sit on them and the sponge pad in the tank tunnel. Don't pull the tank down onto the rubber washers.     

I have filled tanks with water before welding, having first rinsed out any petrol residue. Mind a full interstate tank weights a lot so watch your back! Placed upside down on something soft so as not to damage the paint on the top of the tank, the area to be welded can be manoeuvred to be the highest point so a tiny bit of trapped air behind it won't hinder the welding and the water stops the spread of heat damaging the rest of the paintwork. Welder earth clamp on the other stud. After the tank is emptied of water, blow dry with an airline or hair dryer in the filler opening and pressure test with just a few PSI of air in the tank and some water and washing up liquid on the weld to be sure ot has sealed before a splash of paint and refitting.

Best regards  Al.

Tig welding could be the way to go if you are trying to preserve the paint job.
I had my Matchless tank done by a local engineering firm many years ago and is still leak free and in the same area as yours.
We taped a sodden towel around the outside to act as a heat dissipater and this kept the paint from being frazzled. 
 

Drew at Metal Magic, Eversley Cross, between Basingstoke and reading will sort it for you. 

You can solder steel tanks using plumber's solder with Baker's Fluid or similar as flux.  My Dad fixed my old 88 tank in 1963 and it still doesn't leak!  Of course you have to remove the paint down to bare metal with any kind or repair.  For a cold repair, why not use JB Weld?  Even easier, AND safer!