I still don’t see why the back flange needs to be cut. Cut the bottom of the toe kick box to the right height and to adjust for any irregularities in the floor and mount it to the floor lift the pantry and slide it right into place and screw it to the wall and the cabinet next to it. You have to leave a small space at the top if it goes floor to ceiling. Don’t trust your ceilings are 96” right on the nose measure to see what you have. Then cut trim pieces and cover the exposed ends of toe kick.
Maybe your photos will show me the problem in the back.
Maybe your photos will show me the problem in the back.