As gggGary and FrkyMnky say, solder is the best because it doesn't make the steel brittle like welding will. And, it will flow and find hidden holes in seams.
But, back to the original poster: I had a leaky stock early CB750 tank with perfect paint and no dents. Solder, brazing or welding would have ruined the paint. I used VersaChem "Magnum" Instant Gas Tank Repair from any auto parts store. It is a putty type epoxy, and it worked just fine, although I don't have long-term knowledge. I very carefully cleaned and then made the repair, and when I put gas in it seeped -- but then it sealed. I think it is actually formulated such that gas chemically activates it and completes the repair. In any case, for about $3 it's the way to go if you want to cheap out. But I also agree with what has been said -- from your pics, that tank needs a Caswell tank seal kit, and if you're going to go to that much time and expense, you might as well just find another good used XS650 tank.