I'm with Scott on the intake. Mine was a combination of cast iron block, Dove heads and an original Shelby sidewinder manifold.
As the engine was trial fitted by my builder, he had to remove material from the bottom of the intake sealing surfaces and do some serious port matching to get things to line up.
These top end issues are easy to miss, if the builder is not necessarily an "FE" expert.
My engine did use
oil, but not quite as bad as yours. About a quart in 300 miles. Plugs always looked good so I could only assume that it was burning it well enough not to foul them.
I have learned that it is not a bit uncommon for a 427 to use
oil when they have been set up for racing with the more open clearances. A quart in 50 miles is a bit extreme to be sure. Even in 300 miles like mine is a bit much, but I didn't drive it far enough at one time to worry about it. I did notice that if I drove it hard, it would use more.
I had no valve cover breathers and relied on the breather cap on the
oil fill tube and my "disguised" PCV valve which went from the old 60's vent in the back of the intake to the carb.
Al