[tbirdtbird]

Steel can be soldered but it helps if you have some experience.
Practice on a scrap piece of sheet metal.
The steel must be very very clean and shiny.
You must use muriatic acid as flux; rosin type flux or paste plumbers flux will not work.
Get some 40-60 solder (the second number is lead and you want as high a percentage as you can get)
It is very very difficult to solder upside down so if the leak is on the bottom you may have to remove the tank.
For an iron, you could use a 1200 watt electric soldering iron (massive) or the type of iron that is heated externally in a propane forge. These tools are becoming hard to get.

Of course, never an open flame on any container that has ever had gasoline in it, even if it was 20 yrs ago. There will always be a residue and it will explode.
Of course, you can put dry ice in the tank to displace the O2 and get away with a flame but you will get it too hot and the solder will run out of the area you are working on. The trick is very very controlled heat.

If at all possible have a radiator shop do this work; that is what they do, they solder all day long. Barring that get a lot of tutoring from someone with experience. In the time it takes to type all this I could have had the job done, but as always experience is the best teacher. Miles just soldered up a modern tank at the dealership he consults at. Saved them a fortune.

I am sure there will be the usual detractors but this is what works for me for 6 decades