If you do not breach any of the door opening surround, which would include the rocker panel you can get away without any bracing.
If you need to replace the underfloor cab mounting bracket you might want to do your work with the cab mounted on the frame to get that mounting hole lined up with the frame fixing point. This could be a good start point.
As you rebuild the drivers side you can CHECK DIMENSIONS from the drivers side prior to welding.
The Factory Assembly Manual has cab quality checking dimensions but some are referenced off of a virtual centerline or points external of the cab making it hard to translate for your needs. Still a great reference manual though regarding assembly.
Search this site for a lot of advice on cowl rebuilding.
I am certain others will chime in to provide some good ideas based on their experience.
Good luck and work slowly, rechecking your work as you go.
