I did a similar repair to the common rot points on my Landy bulkhead near the vents as you suggested for your A pillar. I see no problem in doing that and will be a lot easier than trying to replace the entire A pillar. Good luck with the floor pan though as that is would be out my depth not to say I wouldn't have a go though.
Weld your door brace on the inside of the pillars before cutting the old brace out ... Check the inner part of the lower a pillar .. mine is rotten there ..
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I did a similar repair to the common rot points on my Landy bulkhead near the vents as you suggested for your A pillar. I see no problem in doing that and will be a lot easier than trying to replace the entire A pillar. Good luck with the floor pan though as that is would be out my depth not to say I wouldn't have a go though.
Weld your door brace on the inside of the pillars before cutting the old brace out ... Check the inner part of the lower a pillar .. mine is rotten there ..