You may be looking at the symptom of the problem and not the REAL problem. What folks more technical than I would call the "root cause". That would make the oil leak a mere "issue".
The Probable Problem
These tanks hardly ever have issues unless they are not free to vibrate separate from the frame/engine. So the things you should look for first are:
• Bright shiny spots on the tank indicating that the tank touches the fender, battery box, frame, or more commonly, the seat.
• Check the seat to have a full complement of the 1/4 inch high rubber bumpers on the underside to keep the seat from landing on the oil tank fill cap.
• Replace all the rubber mounts on the oil tank (in case they have age hardened) and battery box per the parts book. The battery box has to come out to remove the tank anyway.
• Since batteries that properly fill the battery box are hard to find, make sure the battery is cushioned well away from the oil tank with slabs of dense foam, like "Ethafoam". This is the very dense white or pink cushion material used to pack computer monitors and TV sets. It is expressly designed to fight vibration and shock.
You are correct, the original oil tanks are roll welded at their seams. This is an electrical process akin to spot welding, but instead of tips it uses a set of wheels.
I would highly suggest you strip and braze the tank. The brass will go on at a lower temperature than TIG or gas welding, thereby reducing warpage, and will flow (inside and out) to fillet in all the stress points left by the crack. This will increase the strength of the tank and more importantly STOP the crack.
Epoxies will stop the leak, but the tank may continue to crank since the crack has not been stopped or reinforced. (If you've ever watched the crack in an automobile windshield advance, then you get the mental picture I'm trying to convey.) As the crack advances into new areas, the tank may leak again, and/or flake off pieces of epoxy on the inside of your tank and into your oil supply. Don't get me wrong, I'm a big proponent of Caswell, but your engine oil is not the place to experiment,if you catch my drift.
Hope this helps! :bigt: