[swamp rat]

My thoughts are to work/fit just the lower A pillar now before cutting anything else out, get it cut to length and get the lower holes transferred to it., that A pillar later will become a control point that will allow you to align the lower cab support to it, plus it will keep the floor and support from dropping too low. Ya don't want to weld it right away but it can be clamped in place later with butt weld clamps or tacked temporarily then removed when needed while fitting the rest of the floor and inner kick.