My kitchen sink drain (with disposer) backed up after I tried grinding a grapefruit half. I plunged vigorously but then found the plastic trap that I'd installed below and attached to the metal part to a metal floor tube was leaking badly. I went to Home Depot and they gave me a plastic drain kit, that had a plastic floor tube and a rubber like coupling made by Fernco. They told me to cut the pipe going to the floor, pour drain cleaner into it, snake it, and install the Fernco then the plastic floor tube. I'm guessing the Fernco coupling is supposed to connect to 1 1/2 chrome tubing. This doesn't look like a good way to go because the existing floor tubing connects to what looks like a chrome coupling with two nuts on it (apparently) and it connects to copper pipe that has outside diameter 1 5/8 inch, not even 1 1/2. The coupling that's on there looks like it may be in bad shape with corrosion but can't really tell if it's just covered with stuff that has dripped onto it. So I figured the right move is to cut below this coupling and connect direct to the copper. This is where I need the help. Also, there's a floor to the space in the cabinetry so can these rubber couplings be placed below? The floor will be easily removable once I'm done.