Yeah thats a pretty common mistake. Most people want to replace the rear O2 sensors for these codes but actually this will not fix the problem. The front O2 sensors are the ones that actually need to be replaced. What sets this code is if the switch ratios of the rear sensors start to equal the switch ratio of the front so the first thing that happens is the rear get replaced when actually the front O2 sensors are getting lazy and not switching as often and this is what causes the problem or the cat goes bad one.
There is also a technical service bulletin out from ford to reprogram the computer for this on this vehicle.Here is a copy of the TSB
Some 1997 Crown Victoria, Grand Marquis and Town Car vehicles may exhibit a malfunction indicator lamp (MIL) on with diagnostic trouble code(s) (DTC'S) P0420 (Catalyst System Efficiency Below Threshold - Bank 1) and/or P0430 (Catalyst System Efficiency Below Threshold - Bank 2).
To service, reprogram the powertrain control module (PCM) to the latest calibration using WDS release B28.4 or higher. Replacement of catalysts is no longer necessary to address the most common causes of this condition.
Obtain an Authorized Modifications Decal (FPS 8262 - obtainable through DOES II, 25/pkg) and list the date, dealer number, and summary of modifications performed. Select a prominent place adjacent to the Vehicle Emission Control Information Decal suitable for installing the Authorized Modifications Decal. Clean the area, install the decal, and cover it with a clear plastic decal shield.
Eligible Under Provisions Of New Vehicle Limited Warranty Coverage And Emissions Warranty Coverage
I would recommend having the front O2 sensors replaced and the PCM reprogrammed and this should take care of your concern.