I have had similar results with my 351-W........What I did was add/weld about six inches of thin walled steel tubing, I think about 1.25 inches in diameter to the breather holes,but when you weld them in,set them so they are straight up (vertical) on the valve cover when the motor is installed in the car, also I added a baffle,nothing more than a small piece of Z-71 rear quarter panel in the shape of a V, I inverted the V and put it in the tube from the bottom and welded the two ends in the tube,solved all my problems.........
Got the idea from the original 289 Comp car vavle covers.......what was happening was the vent worked great,but after shut down a little
oil/mist "pooled" in the K&N breather and since it was at an angle it dripped out the side of the breather element on the headers instead of draining straight down back into the engine........
I have seen pics of the old 289 comp car vavle covers,but can not find one now,maybe computerworks can find the photo and post it,it will give you an idea of what I'm talking about............
notice the photo FUNFER2 posted and you will see the vent tubes with the breathers stand straight up,so things drain straight down back into the system and not all over your motor........mine are very similar,except I welded a tube to each vavle cover coming straight up and use similar breathers.......
the other thing is to route the vent to a catch can...............
David