409 used an oil fill tube in the front of the manifold with a vent on top of it. It also had a PCV system using the road draft tube in the back of the block with a hose attached to the rear most carb to suck in fumes and burn them. Without these two things your motor will pressurize the crank case from blow by (really prevalent with a 409 due to uneven weight distribution on the pistons) and pump oil out of any opening it can find. If it can't find a big enough opening it will blow out all of the seals in the engine (oil pan, rear main, front seal, valve cover, and china rails) to make an escape route for the blow by.
If you have these things on your motor it is time to call your reputable engine builder up and ask him why it has so much crankcase pressure. I would also ask for a leak down test to check ring seal in case he broke a few installing the pistons (409 requires a special ring installation tool due to the seventy four degree angle the deck makes with the ring pack, which is aggravated if you used a truck block with the holes cut into the block above the top of the ring pack to drop compression).