J B Weld will work for awhile, but eventually it will seep, especially if the crack continues to run. Yes you can weld the pan without removing it, but it does require certain precautions. I had a hairline crack in one of the factory welds on my Milodon pan. First I let it drain for a good 24 hrs while tilted towards the drain plug. Next day I tilted it the opposite direction [ crack was on drain plug side ] so the oil would drain away from the crack. I have a friend who is an outstanding welder come over with a small TIG welder and we used a "T" fitting for his argon supply and made a fitting that we could put in the pan's drain hole. By running argon, a non-flammable gas into the pan we had no problem with flash fires and he did a masterful job of welding the pan.