I have a cracked drivers (left) door just forward of the mirror (pictured). The crack allows the window opening to widen and lends to general instability. I am fairly positive that it manifested from 30 years of door insulation that was too thick. My plan was to clean it up and weld it back together. A series of spot tacks from the outside that can be smoothed down and repainted. I am not a body guy by any stretch of the imagination but not unfamiliar with a welder (MIG in this case). Is there anything that I might be missing or not understand hiding here? Thanks for any tips or suggestions.

That is a common place to see cracks. I think mig is a reasonable choice