I don't think you need anything else. The two floor pieces should screw right to the original nut plates, no mounting flanges should be necessary.
Just understand that is an old post of mine and those floor pieces in my pictures were made by me BEFORE Van's changed their design to two floor pieces, so I have never seen the floor pieces that you will be getting.
Best luck, it should go easy.
RV-12 #202 -- Flying