This photo shows how the reinforcement beams are placed on the underside of the roof. The typical landaulette conversion removes the roof panel up to, but not including, the first beam counted from the upper edge of the rear window. This is the beam above the C-post.
The beam is just above the front edge of the seat of the bench, so the passengers won't be able to stand upright through the open roof. However, that beam cannot be removed without compensation by adding an extra reinforcement via the floor.
The photo comes from the gallery-section of the restoration of Zoran Mitrović's DS420.
This restoration was conducted by the Avala Auto Centar workshop in Serbia.