First step to restoring my 1974 Duster; Front End rebuild (coilovers needed?)

It’s pretty unlikely you broke that bolt, and the bolt just goes through the spindle into the lower ball joint so there’s nothing else to really break. The bolt isn’t retained anywhere, it just slides through. The nut probably just hit a piece of junk or a rough thread on the way out and started spinning the bolt, doesn’t take much once the tension is off.