I've known of people fixing the crack of doom by either welding the crack, reskinning, welding in a patch that either replaces a rectangular area that goes on either side of the crack, or welding the crack and welding in a rectangular reinforcement behind the crack.
The way to make any of these repairs last is to adjust the vent window so that excessive pressure isn't exerted against that area of the door skin. That area is about the weakest area of sheet metal on the door and if the leading edge of the vent window frame is lined up so that the windshield frame is pushing on it when the door is closed, guess where that stress is going and what it's going to do? Yeah, welcome to cracktown, your MGB.