Your on a wild goose chase. Any brand LT header will eventually set codes. The cats are now too far away from the engine to work within every single efficiency parameter in the OE calibration.
Due to varying driving conditions, some may set codes in 2 days, some maybe 2 months, maybe 6 months... But no way will aftermarket cats, set this far away
from the engine never set a code.
The OE cal is set to precisely monitor the cat efficiency when it is way up next to the engine. You bought SLP stuff, give them a call. You will hear the words - tuning device.