If you want a 6" bit I would suggest a strikemaster lazer bit. Much better blades than the ion. They are about $140. You will have to modify it a little bit as it won't be a snug fit.
You'll have to drill a new hole through the auger bit, since the hole in the bit will be too big. You'll also have to wrap the shaft of your powerhead in some tape or something as it will be a little loose fit to the strikemaster bit. Then just run a pin or bolt/lock nut through the powerhead and the new hole you drilled and you are ready to go.