I don't claim to know anything about big blocks, I have a buddy who does, I just called him and he had a similar issue, motor was built no top end oil, gave up on the motor & sold the car with motor apart it was taken to a machine shop in KC and they rebuilt it, same problem, never got fixed. I asked what he thought it ended up being and he said some bbc take grooved cam bearings to lube the top end, or some aftermarket distributors castings block off the oil passage incorrectly not allowing oil to the top end, he firmly believe his issue was incorrect cam bearings, he said not all bbc need the grooved. Just the info relayed to me....