68 Dart Rear Pillar issues for repair
To fix what you have, I would remove the chrome drip rail trim as well as the windows and interior trim parts. I would then remove all the paint and lead filler which may be present. After that, it would require some inspection to determine the extent of the damage. It is impossible to describe how I would fix it as each issue like yours is unique but it could be as simple as clamping the crack back tight and welding it, or most likely I would see if I could get a patch in behind it to reinforce the area. Drilled holes in the original metal with a plug weld to the reinforcement on either side of the crack and then a weld down the crack to the reinforcement would be another method. I've seen stuff like this on lots of different car models. The reason for the crack is the sharp corner which causes a "stress riser" where stress can not "flow" around. The Lockheed Constellation airliner had rectangular windows and they had some catastrophic cracks that propogated from the window corners resulting in crashes. The fix was to replace the windows with rounded corners.