The crack goes all the way across? I would not ride it anymore. And JB Weld will do no more for the strength of the metal than a piece of electrical tape.
If there is a crack going all the way across the BB shell, it is already flexing when you ride it. There's a lot of stress in that part of the frame. Aluminum doesn't like to flex and it's a sure bet there will be more cracking soon - if there isn't already. If it cracks across the top of the BB shell now, it very well could split open and spit out the crank, causing you much pain.