First we would recommend removing any pre-existing tub or shower as the substrate must be clean, even, and load bearing and any leveling must be done prior to shower tray installation. Measure the installation area to determine the proper tray to use. It appears that you may need the 38" x 60" off-center drain kit KSK9651525S and then just cut the tray to size prior to application; ideally, the tray is cut equally on all sides to ensure a consistent perimeter height.