Could be a few things but my first guess would be a head gasket. Water is getting into the oil somehow and thats one way it can. You also want to look at how the water going into the block is regulated because the block cannot take the full pressure from the jetpump. Usually you want 15 psi tops and there are many ways guys acomplish that.
Next you can check the timing cover. The 460 has a funny bit on the casting that can possibly break thru, if it does there is potential for a leak into the timing cover and then into the pan.
Can you get a bit more info to us and or some pics? did you pull plugs yet to see how they look? usually if its a head gasket you can tell what cylinder(s) are not firing
Cheers