How important is it to weld a brace on inside of cab before..

[StingRay]

Yes sort of. If the cab was dimensionally correct before then yes and if not then sort of. I've found that when these cabs rot out the rockers, corners and floor that the whole B pillar can spring out of whack. I've only done a couple cabs but have had to lever the B pillars back, adjust the A to B pillar spacing and cross measure the cab to square it up as best I could. If you brace in a way that problems can't be corrected then you might not be doing yourself any favors.

I adjusted my doors, fenders, cowl and hood as perfect as they could be done before starting and then made corrections to the cab where I felt it needed to done as I made the rust repairs. I readjusted all of the panels time and again through the process removing and installing as needed. It takes a boat load of time

Oh and yes I braced the cab but I did it in such a way that I could adjust it if necessary.