Each roof pillar must be fitted alone, and also together with each other to make for a good fit overall.
I raised the roof on wooden blocks temporarily to do this work.
You can see the rough cut on the C pillar from a cutting torch. What a mess.
I drew a pencil line across it as a guide for a clean cut.
After taking a circular saw to the pillar, it now looks straight and can be mig welded to without difficulty.
We will make a pattern out of cardboard first to copy to sheet metal later for a section to fill in the gap.