cracked headsWelcome to Allpar. I believe that they were released in 2003 1/2 (January 2003) as a 2004 model. There is no 2003 Pacifica. If the 10th character in the VIN is a 4, then it is a 2004.
As Valiant says, the 2004 Pacifica takes the 2004 3.5L. The 3.5L in the LH is also different than in the CS.
What is wrong with your 3.5L.