I am replacing a toilet on a concrete floor in my basement. I plan on tiling the floor so I want to tile first and then replace the toilet so it sits on top of the tile. I removed the old toilet and found a cast iron drain pipe, a mangled metal flange and some type of rubber stopper the previous installer must have used to fit the toilet on. I know the drain size is 3" because when I removed the stopper, I inserted a 3" piece of PVC pipe into the drain to check the size and it fit perfectly, nice and snug. Can I attach a 3" toilet flange directly to the cast iron or are there other considerations that I don't know about? What would be the best way for me to go raising it high enough for backerboard and tile? I'm using Schluter-Ditra for underlayment. Thanks for any help.