If I were doing it, my first choice would be to get that same section from a non rusted donor car.
Remove the rusted area, trim the donor piece to fit and weld it in. If only the inner part of the windshield frame is rusted, not the entire pillar, I would only replace the inner part. Removing the entire pillar may allow the roof to become misaligned. By leaving as much pillar in place as possible, the alignment will be maintained.
If you cant get a donor piece, then just cut out all the rusted metal, trim square. Then get new sheet metal the same thickness, cut and bend it to match the contour of the pillar. You may have to put it in in short lengths because of the compound curves. If you heat it red hot as you put it in, you can work it a lot in multiple directions and match the curves perfect. Grind smooth to finish. Take your time and with just a torch and a lot of patience you can get it done.
The donor piece will be much easier and quicker.
Good Luck.
