I would be more concerned with the rot down there by the mirror mount, which will probably compromise the door hinge area. Before you start whittling on metal anywhere, remove the door and windshield and install some temporary braces to maintain the shape of the door and windshield openings. I'd suggest looking for a donor cab with a good A pillar and make a cut that includes a section of firewall and roof instead of patching on the pillar itself.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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