BB OIl leak

[stope4]

O.K. I've done everything I know possible to try and stop an oil leak on my 71 BB 402. I replaced the valve cover gaskets, oil pan gasket, intake manifold gaskets, rear main seal, and distributor gasket. I used the black gasket maker along the front and rear of the intake manifold. The leak is occurring somewhere above the oil filter. It's not coming from the oil guage line, or any of the above areas where I replaced gaskets. I know this cause when I rub my fingers at the intersections of these, I do not get fresh oil, it's dry crap. Is it possible the heads could be leaking oil out between the head and block? The vehicle does not burn oil. I noticed theres a screwed in plug on the back of the drivers side head. Could this plug leak oil? Any advice is welcome. Short of pulling the heads, not sure what's next.