If your sills are not rotted out that's a good thing because they are heavy structural components and will hold go a long way to holding the cab in alignment.
If you want to be on the extra safe side, temporarily weld two pieces of angle iron in the shape of an X inside the door opening on the side you are replacing. Do one side at a time. Tack your parts in, remove the braces and hang your door to check. If everything checks out finish welding with the door on and latched. Do not just start welding at one end and run a continuos bead. Run beads about an inch long skipping at opposite sides. Welding exerts stresses on metal that will surprise you. Take your time.
The weight of the door puts stress on the opening. Leave it off until you get your panel in and tacked down very well.
I wish mine looked as good as yours.
