Do one side at a time, cowls first. If you are not cutting through the sill or the hinge pillar you won't have to brace anything. Your biggest issue is being sure the contour of your cowl matches the contour of the door so test fit before welding.
Expect to tear your hair out getting the inner-to-outer cowl and inner cowl to fit. I've yet to see two people attack this problem the same. It's a mess so be prepared.
Once you get your cowls welded up deal with the floor making sure you don't mess up the dimension between the sills. Then do the firewall.
Rear cab corners last. After all that other stuff, it's a pice of cake. Be sure the contour of the cab corner matches the door.
Hope that helps.
