I'm not sure that LT 13 has all this functionality, but if not this would be a great time to upgrade.
Display the 3D model in plan in Datacad. Then 3D Menus, Section, Cut Line. Place the Cut Line where you want, and the direction of the Section - Select 3D Knife, then New (for New Layer) Place the cut model on this new layer, have only the new layer active and invoke O2C.
One final comment. Make sure you have modelled all internal walls, doors, furniture etc. before you embark on this journey.