There's probably nothing better than solder, but it seems if you scuffed both surfaces(BRASS bracket and tank back) with 600 grit sandpaper and cleaned it with alcohol (remove all residual oils) JB should hold it.Might check your rear seam of the tank for leaks before doing it. I have built mufflers with JB weld (alluminum to alloy joints) and holds up well. If you solder, pressure test it with your empty fuel syringe, plug the vents, and submerge it in a bowl of water.
Just my experience...