Have you had the LTFT long term fuel trims checked to see if you are getting enough fuel? You probably need more fuel with the cats gone, same as if you had installed headers. Is the rear O2 sensor disabled or are you using a simulator? How about fuel pressure. Have you checked it at idle as well as under a load? I don't remember if the fuel pressure is supposed to be maintained at 53 or 58 lbs at all times. if you drop off pressure under load, your pump is weak. The PCM compensates for richening the fuel mixture under acceleration or low vacuum situations. That is up to a point. I believe the fuel trims max out at 25 and if you're still lean it won't go any further than that. I'm sure you've already replaced the fuel filter since you've been fighting this for a while. If you have a blown head gasket, that's most likely you main problem. Believe it or not, the knock sensors have been known to hear a blown head or exhaust gasket.