l have a 97 Towncar and I've recently gotten at the same time a p0420 and p0430 code. Is that odd to get both together and could the problem be an o2 sensor or something else causing these codes? l did just replace a PCV valve which could have opened a vacuum leak or be a bad valve.

check your egr valve that could be worng some time that give that code

DTCs P0420 Bank 1 (or Y-pipe), P0430 Bank 2
Monitor execution once per driving cycle
Monitor Sequence HO2S response test complete & no DTCs (P0133/P0153) prior to
calculating switch ratio, no SAIR pump stuck on DTCs (P0412/P1414), no
evap leak check DTCs (P0442)
Sensors OK ECT, IAT, TP, VSS, CKP
Monitoring Duration Approximately 900 seconds during appropriate FTP conditions
(approximately 200 to 600 oxygen sensor switches r collected)
TYPICAL SWITCH RATIO CATALYST MONITOR ENTRY

did you reset and clear your codes after your replaced the PCV valve? l would reset and check codes again.

First, where did u purchase ur PCV valve? l have seen aftermarket PCVs draw the incorrect amount of air (too much) & cause many different problems for their unwise owners. l recommend that u get one from the dealer & replace it before chasing trouble codes, especially Generic (P0xxx) ones like u have reported in ur question.

Generic codes r really a waste of time, it was an attempt by the law makers to standardize driveability troubleshooting across all makes -- WHAT A JOKE! it was way better before!

There is a technical service bulletin (TSB) from Ford about these codes (P0420 & P0430 Catalyst converter efficiency below threshold) set in 1997 Town car, Crown Victoria & Grand Marquis V8 4.6L engine.
The power train control module software needs to be updated to the latest calibration. The service is covered under emission warranty but ur car is 11 years old so u may need to pay for test & software update.The original TSB number is 03-24-5 release date: Dec 8 2003. l would give a Lincoln Mercury or Ford a call to ask them about the update. They can look at ur vehicle warranty service history.

If u have air leak because of the PCV valve, ur car PCM will set many other code first (fuel trim code, oxygen sensor code, misfire code.). Catalyst converter monitor will not run unless all other monitors (except EVAP) completed & passed.