Sounds like a band aid fix. As you know the frame is structural and any hairline crack is going to get worse with time and mileage. JB weld ain't going to restore stability to that failed weld, it may not even fix the leak. The correct fix would be to drain the tank/frame then tear the bike down to where a welder has sufficient access for a good tig weld.
If I was in a pinch far from home and needed to limp her back, I would drain the tank then use aluminum epoxy on the crack with masking tape over it to hold it in while it cured. After full cure time; put 1/2 tank in and lean the bike towards the repair and check for leaks. If no leaks then fill er up and go.