(Above) I have welded in (but not dressed) the surface of this corner bracket (taken from the Goat Lady's van). It isn't perfect but it is a heck of a lot better than it was. It was tricky because it needed 3-d adjustments and had to be right on 4 mating surfaces. Considering how awkward this was, I don't think I've done badly.
(above) I braced the windscreen so that the gap did not widen and the frame sag, when cut. Then I carefully cut away the rusted section of the A-pillar. Then (below) I inserted the replacement section. What you have to imagine, though, is that the replacement part includes the upper left internal corner, mirroring the right hand one you can see at the top of the page. Filing it so that it fit in 4 places was tricky and accounts for the discrepancies at the front.
I will need to clean the rusted surfaces at the front before I can weld the part in place. I may put a small over-lapping insert piece in to brace it....but fitting the part once the insert is fixed in it will be difficult.
I am quite pleased with how this is going because when this is done, all of my structural rust problems around the scuttle will have been solved.