Someone (Philips/NXI?) has an IC that can be used in pairs to extend it over longer distances.
Can't you power everything with 3V3, including the Arduino? (In this case, the safer frequency would be 8MHz.) So you can get rid of the level shifters.
20 feet is too far for reliable I2C although I know people do it.Search for P82B715 and PCA9600 for a start.