You just confirmed what I know to be true, crooked or leaning cabs are rarely rear mount problems. Here is a link to how I fixed mine, just follow the pictures.
https://www.flickr.com/photos/advance-design_parts_co/8129129278/in/album-72157631867396207/Most of us that go into serious cab surgery replace the following:
One or both floor sections usually above and below the bend line.
Inner and outer lower cowls with center section.
One or both front cab mounts (formed sheet metal channels see pic).
https://www.flickr.com/photos/advance-design_parts_co/8129107861/Re-bush and refurbish rear shackles.
All of these sections require pretty good welding and fitting skills or the willingness to learn. Parts are available from several vendors, none are perfect, most work. I will add that I replaced one complete side before removing anything from the other side. My cab came out really well doing this, others cut and brace. I don't like welding all that additional support in (with subsequent clean-up) if not needed, that is a decision you will have to weigh.
