I have had the same problem. The dealer I took my '98 GTP to showed the same P0300 error. They said it could be the 3.4 pully due to the added air. I think they were just guessing. I don't know what the fix for it could be but let me know if you find out. Have you ran the car with the 3.8 pully on? Does it run fine like that?
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3.4" pully, aux. trans. cooler, SLP CAI, Jacobs Coils, Jacobs wires, slot rotors.