Cataylic converter

I own a 2002 3.4l and need to replace the front cataylic converter. The price I am quoted for the part is around 225 yet the dealer quotes 1200 which I am assuming is from the engine back. Do I have to replace from the engine to the converter or can I just replace the converter? Considering that I see that it just bolts on it does not make since to replace the whole thing upfront.

I am assuming that I have to replace cat because I replaced the exhaust from the rear cataylic and O2 sensor and now get the check engine light with cat info on it. Can't replace front O2 sensor because there is really nothing left on one of the studs.