A couple of thoughts:
-When replacing the seal, did you coat it with oil (just like the oil filter gasket)? Did you follow the correct torque specs and sequence when tightening the seal plate?
-Did you do the same for the oil pan? Was the gasket reused or replaced? Some people put some silicone on the corners to help out with the sealing as well.
-Is the leak located below where the rear main is? Not sure where the 05 filter is located, but I know some Mod engines have the filter located out back. If your is, check your filter. Also, are you sure the fluid is engine oil? Other possibilities are trans fluid (red) or clutch fluid (clear). Valve cover leak could also be possible towards the back.