I can sympathize with you for wanting to try a quick fix to get you to the end of the nice weather. If I were going to use JB Weld, I'd drain the pan, then jack up the side of the car with the crack on and leave it for a day or so, so the remaining
oil in the pan runs away from the crack. Then follow Specialk's advice. Prior to welding, drill the ends of the crack as outlined above and V it out. I'd TIG weld it if possible.
Good luck.