I also approached mine first DS then PS completing all work on a side before moving to the next. If you cut and weld as you go you should have no effect on the cab. One thing to keep in mind when you replace the outer cowls is to mount the door to check the clearances you don't want to big or too small a gap. You will find that the panels are not the same as your floors there are differences. I cut the new panels to account for things like missing brackets around the pedals the originals are better the small little hump in the center that did not appear on repros. I cut where the originals were in decent shape and did not remove more then necessary. You will note that some panels go to the break line in the floor and others have that line. I would suggest buying the full panel with the break and upper section in it. That area around the break would make it very difficult to weld since it is usually very thin and rotten there. Take your time measure, measure twice, and have fun I really enjoyed this part of the restoration as I sought to make it better than new.
