Ex- 6BT Square Body Man
Bingo !Did you inspect the wiring running out of the fuse block? There have been some rub through issues coming from the fuse block
Checking resistance in the wire for the FRP back to the ECM at various points.
I found that when I got to the connector to the ECM it would have resistance to multiple pins..
So I tore all the sheathing and tape off that path from FRP to ECM, separated the wires, couldn't find anything obvious.
For giggles I hooked enough wire to fire it back up and boom CEL is off and no more codes.
Now what do I do ? Tape and sheath harness back up a section at a time ? And test.
A new harness is $800 from Lincoln Diesel. Discontinued from GM