Scott:
This is a pretty dodgy subject open to a lot of opinionated debate. From a fabrication standpoint the crack itself has to be terminated before welding. This is done by drilling a small hole at the extreme ends of the crack so that the end of the crack so to speak terminates in the area removed by the drill bit. Then remove any oils to prevent contamination of the weld (gas or electric) and then reweld the parent material with mild steel rod.
I really suggest removing the pan to do it properly. Harmonics will continue to make the crack grow longer.
You could apply some sort of material (epoxy, silicone, etc???) to buy you some time and stop the leak, but it would only be temporary. Just grin & bear it
Rick