I'm in the (slow) progress of changing my door step and boxing plate as part of a full rebuild. My plan was the replace the door step first and use the original boxing plate edge to help align the panel. Weld it to the a and b pillar, then cut out the boxing plate, replace that and then weld the door step panel and the boxing plate together.
I braced the shell before I cut out the old door step, but the brace across the door is welded to the boxing plate. I'll have to cut this weld to replace the boxing plate, but what should I temporarily weld the brace bar to? Does it matter given the door step will be in ( although not fully welded). My fist thought is to add a little diagonal bar and weld it to the flitch?
Picture to help

It not clear from the pic, but the floor and inner sill is rotten, so I can't weld the door step to that, until I've replaced it.
Thanks Chris