P0420 and new cat

[Radwaste]

I've been troubleshooting a P0420 these last couple weeks. So far I've cleaned the MAF, done a full tune up, checked for intake and exhaust leaks, and replaced the post cat O2 sensor. All no dice. My post cat O2 sensor voltage output hangs around 0.05V to 0.3V and doesn't track much with the upstream O2 sensor, regardless of how I run the engine. The upstream O2 sensor channel checks with the other one. Other ideas?

 

So now I'm in the market for a new cat. A little research puts pricing anywhere from $50 to $600. Any advice or history of replacing the stock catalytic converter with an aftermarket? Is is a bad idea to only replace one of the two? What's a reliable brand that wont just end up giving me a different code?

 

Any help would be great. Thanks.

[Radwaste]

An update: I'm in the process of replacing the exhaust starting with the cats. I've decided to go with Magnaflow, 2.25" out of the cats and a single 2.5" out of the muffler.

 

My question is concerning the fit. Those who have replaced the cats with aftermarket, was an additional high temp gasket material cutout required to get a good seal between the stock exhaust manifold and the aftermarket catalytic converter?

 

All I see is a metal ring gasket that doesn't look quite up to the task.