A new tile floor has resulted in a finished surface 3/4 of an inch above the existing 4" cast iron flange which is seated in the concrete slab floor. The inside of the cast iron pipe - where it is level with the flange - is slightly irregular (lead?) and may make it difficult to insert a rubber gasket in to the 4" pipe. I plan on installing a new low flow toilet and would appreciate advice on selecting an extender and gasket.