Sorry to say, but from your pictures it does look as if a new tank is going to be needed. Often steel tanks rust on the top (we were for ever doing them on the Morris Marina I remember for just this reason - didn't look too bad in the car but when you dropped them out the top, which you couldn't see, was covered in damp road debris and rusted through like a pepper pot!)
You used to be able to buy a thick paint on substance (was it called "Petropatch"?) which cured a bit like glass fibre. Many years ago I used it with some success but after a while the tank just started leaking somewhere else!
Welding petrol tanks? A very risky undertaking. The only one I've ever done was when I was very young and ignorant. I had a single pot 350 AJS - which I loved dearly. It started weeping slightly, which is not good right above the engine! I removed the tank and used the college (I was still at automotive college) steam cleaner to thoroughly steam out the inside. The welding went without incident and the leak was cured but the welding instructor came in half way through my endeavours and nearly had a heart attack when he saw what I was doing! He calmed down when I told him it had been fully steamed out and anyway was only a small tank but I got a very stiff lecture about the potentially horrendous consequences of welding fuel tanks!