Here's my version of the A-pillar repair. Mine's a '69 911E. I had a pretty good bit of rust running under the windshield seal and it had eaten all the way through.
As described earlier- Cut out the offending part
Shape and fit a patch (much filing, grinding and fitting on my part- but I'm getting better)
Weld it in (sounds easy, but I found this spot a bit of a challenge because of where I had to make the matching point and the difficulty of getting the MIG tip in there (even with the glass out)).
Grind it clean and paint.
At least that's my story.