P0420

Hi Guys new to the site want to give you a try, it's a goody. My wife has a 2001 chevy camaro, 3.8L, 102,000 miles. We keep getting the code P0420 which is catalyst below range left bank.
I replaced the converter at 83,000 (had come apart inside) with a high flow universal at my local muffler shop. The factoy one cost $550 plus my old one, the muffler shop cost $175, the walker one cost $358. Ok here's where the fun starts. Everyone told me that it had to be torch welded because of the material the factory pipe was made of. Ok i'm a mechanic but I don't do exhaust for the reason of I don't have a lift yet. Of course I had to take it back several times for the weld breaking. And allowing o2 into the system and being noisy. The p0420 continued to show up all through this. About 2
months ago I order a replacement from walker muffler (the replacement goes from manifolds to the next section behind the trans. Of course it was made wrong and wouldn't fit. While I had it off I noticed that the 3" section of pipe going into the converter was steel, not alumnized, so I welded it with my mig.I had already replaced both o2 sensors in the manifolds and the downstream one also. Now I have an airtight system, 4days later the light came back on with a p0420. Any ideas?