If you were running rich\lean I would expect to see those codes as well P0171\P0174 I think. The lack of these codes leads me to believe that the gasket leak is a bunch of BS.

99 CSVT Green/Tan # 84 of 2760 Built on Wednesday, September 30, 1998 15.376 @ 92.00 MPH Stock 75,000Km 96 Contour GL 2.5L V6 ATX 170,000Km