You definitely want to install your doors before you do any welding. Lots of screws will hold everything in place as good as the original spot welds did.
The number one reason to screw the panels in place is so that you can move things so that the doors will fit.
One would be wise to adopt the notion that in the process of replacing cab sheet metal, one should start with the doors and build the cab around them.
As long as you have a solid "ring of death" (as I like to call it), that is, a sound door opening comprised of the rocker panel, hinge post, lock post and top post, and your sheet metal panels are screwed into place, you really don't need bracing any more.
