Weld it, man. JB stuff is okay for non-critical stuff, but this ain't it. Check in the bottom of the tank really well. I use a Hobo Freight telescopic camera to scope it out inside, especially on Mobys. If there is bad rust or pitting at the bottom, maybe have your welder put some bracing around the bottom too where the tank ends. I've seen some nice external plates welded onto frames that blend in nicely with the lines of the frame, alternative to a bolt-on cross bar.