Hi there,
Why didn't you use the correct O2 sensor for your engine? Is it possible that the engine never goes into closed loop mode? It might be running on the default settings, which means that it would be running rich all the time, as your test data clearly shows. (I could be off base here, not familiar with your engine management system, but that's the way the setup in my '82 works.) A new catalytic converter would just hide your problem for a week or so, then it would become fouled and fail. You need to find the cause of the excessively rich operating condition.
Here in Washington State they test at idle and at cruise. They used to use a dynamometer to load the drive train at cruise, but have recently discontinued that part of the test, and just run the engine at 2400 rpm now. If the engine is running properly, it will pass both tests easily.
Good luck with it!

Richard Wooldridge
'82 300D/4.3L V6
'77 Jag XJ6L
'77 280Z
'89 Astrovan
'94 Ford Taurus
'85 GMC S15/87 Firebird engine
'65 Clark Cortez/'78 Caddie 425CI engine, FWD drive train
(currently licensed vehicles only)