Yes, the passenger side pillar is welded to that brace. All the bracing I put in place before I removed the top half of the cab from the original floor. Here's a pic:
The drivers side A-pillar ended up having to be replaced completely (which I wasn't expecting when I braced the cab) so when the replacement pillar was lined up along the windshield area it didn't line up at the brace anymore. It didn't pull inwards from welding, it was always like that, even when it was just clamped into place.
Good call on the floor though, I will probably measure the widths to confirm.
The wing panel is what I call the curved inner panels with the vent holes.
Moving the drivers A pillar against the brace isn't too hard, I can squeeze it together with one hand. It's clamped into place now with vice grips.
Rough sketch of what I think is going on:
