Excessive pressure - based on all your efforts to date.
PCV and breather condition?
Condition of crank snout?
The front pan to cover seal design is poor at best.
Most builders I know rely on the Right Stuff:
http://www.permatex.com/products/our...he-right-stuff
Mating surfaces cannot be too clean.....I have a new Milodon pan and cover, engine assembled in the shop with the Right Stuff and it still weeps a little oil.
Getting that area to seal with the motor in the car is tough not to mention a real PITA.