I had this problem with a severely rusted and repaired cab. You absolutely need to mount the cab on a strait frame and adjust it as specified by the book. I spread out the opening at least 3/4" without buckling anything. I spread the opening by bolting a big c-clamp to a piece of square tubing in such a way as to use the screw of the clamp to do the spreading. I went from hinge pillar to latch pillar. Even if you do buckle something, I think that would be easier to fix than the heat warping of the door, with all of the welding of the entire length of the door, is likely to cause.
