Are the duties of the O2 sensors identical between L30 and L31?
Are you using an L31 tune on the computer?
I have not researched this. Look in the service manual for confirmation. My suspicion is that both L30 and L31 have four O2 sensors, but they're not intended to perform the same functions. Specifically, the L30 has three sensors pre-catalyst, and I'm wondering if the L31 has two pre-catalyst, and two post-catalyst, with two catalysts needed. My '97 K2500 has two cats, and two downstream sensors, but it's a 7.4L not a 5.7L.
IF (big IF) that's the case, the computer is expecting two very active sensors, (pre-catalyst) and two relatively dormant sensors--readings tending towards very lean--post-catalyst.
The quick, non-invasive test of the catalyst is to find out if the output of the catalyst is hotter than the input. A few seconds with an infrared "temperature gun" would tell that tale. Another way is to assure that the downstream O2 sensor(s) is always reading a fairly stable, lean condition.
You could remove the catalyst, but other than verifying that it's not plugged...I have no idea how you'd verify that it works, off-the-vehicle.
California requires CARB-approved catalysts, which is to say the REALLY expensive ones. Either direct OEM replacements from the vehicle manufacturer, or certified aftermarket replacements of the same efficiency, and carrying that CARB approval. If there are exceptions for folks moving-in with non-California-compliant vehicles, I don't know about it. Research needed.
Re-think your move to Commiefornia.