To do a belt and braces full replacement of the boxing plate involves diconnecting the bottom of the B-post. This is seriopus stuff and shouldn't be tackled lightly if you ever want doors to fit properly in future.
If the boxing plate is only rotted at the bottom, it can be replaced by letting in new metal (thick steel, welded properly - NOT just plated over)
However this depends on whether the boxing plate is solid behind the B-post. If it is then you can consider having it repaired the 'easier' way.
Oh dear I have heard that corrosion in 4 doors in this area can be a real so & so to sort out.
If the boxing plate is mainly ok apart from bottom edge and the areas behind the bottom of the C-post [the bit of rear arch you mention] then it is a simple job to cut it away and let in a section of thick steel here.
The
hard job is to make a good repair of the C-post itself after you have cut it away to get to the guts of the sill/boxing plate. However repair sections are avalable and although they are not usually perfect, it makes the job a lot quicker than trying to hand make a repair section!
On my 54 the inner sill, inner step, boxing plate, wheel arch and bottom of C-post were all rotten, so at least I had good access for welding when the rot was cut away

After removing the inner sill (officially that is part of the floor) I could then get access to repair the inner step where it passes the rear seat box section. Most repairers will ignore this, however this is an important structural area and is right next to the spring hanger.
I'm just going to look for some pics of my car resto website... brb
Right - first pic montage shows cut away allowing repair of 'hidden' inner step area and the subsequent letting in of more metal.
Second pic montage shows the rest of the areas (there's a lot of sections meeting in this corner

)
in the top left you'll spot that I've put a thick piece of steel to the boxing plate where it is 'hidden inside the C-post. I used thick steel to replace the bottom of the boxing plate but I was going for strength, not originality, and also with time constraints there was no chance to replace the boxing plate completely
My C-post repair section was pretty ugly but it wasn't done for looks. It could have been ground down a bit and filled to make it look good but I only had time to hammerite it and carry on with the next job.
Most of this area is hidden by the kick plate anyway and when the door is closed there is virtually none of it on show.
The wheel arch repairs and seams came out really well, but were hidden by the wing
Ray. MMOC#47368. Forum moderator.
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where to break down next?
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