Oil leak, valve cover or heads/head gasket or neither?

[69ChevyRazorback]

My old 69 is drinkin about 1 quart of oil every 2 weeks. I have a rear main seal and a front main seal i believe, along with a leak somewhere on top of the motor. After running my truck, oil pools up on top of the intake manifold on the left side of the carb . the oil pools up around where the bolt on top of the manifold is. Also I just changed spark plugs and the third spark plug back on the left side was completely fouled and it looked like oil ran down on it which fouled it. Without looking, do any of you guys think u know what this is or have had similar problems??? im just not sure how to make the oil stop leaking without a complete rebuild. i also use lucas stop leak very often.