Not sure about your year truck but my suggestion would be to fit the repair piece so that you have the gap and contour matching the door. Then work the rest of the cab/jamb to line up with that.
If I read correctly the jamb gap was previously "corrected" with plastic filler. This something you must avoid, the edges of panels should always be metal so keep that in mind.
HERE'S A PICTURE of the kind of "clamps" I used to keep the surfaces flush.
DETAIL SHOT of the clamps.
Good luck!
