A-Pillar repair

I have a 74 Dart Sport that came with a qtr vinyl top. I've removed it and found rust on the driver side, along windshield chanel and on upper a-pillars. Looks like the a-pillars were filled with lead. Has anyone come across these problems and how do I go about fixing them. I'm just learning how to weld and I don't know how to fab pieces yet. Any help would be appreciated.