[Flat Six]

IIRC, an earlier thread of yours said you cleaned your ICV? Might want to double-check its operation - momentarily jumper 12v to terminals you should hear a click as the valve opens/closes.

Based on what you've already done and the idle/backfire symptoms you note, sounds to me like an ignition/ignition timing issue. If it were me, here's what I'd check (or replace) if you haven't already:

1) distributor cap and rotor
2) check ignition coil (I assume you replaced coil lead when ignition wires replaced)
3) pull plugs and check condition
4) run w/borrowed - and known good - DME to isolate whether that's the issue
5) check clearance of crank position and timing sensors
6) re-check output of crank position and timing sensors while moving their lead wires (can get inconsistent readings if insulation on wires is heat damaged/cracked), these can give occasionally good readings (false positive)

Try re-testing the O2 sensor, making sure your're measuring the output leads and not the heater lead. If you're still getting 0.13 volts your O2 sensor has likely gone south -- should fluctuate. After re-checking voltage and finding constant reading, unplug O2 sensor and drive the car a little. Won't hurt it; just will run in 'open loop' condition. See if some of your idle/backfire symptoms go away; will help confirm O2 sensor as a cause.

Be aware, though, you might have multiple issues contributing to this running condition.

Hope this helps, and good luck.

Dale