Oddly enough I have this same issue and I am also running a 9-s centerbolt style SBC mine is a 305. I have the exact same leak at higher rpm I thought my pcv was nto working but I canged than and put in new breathers to no avail. Please let me know what you find to stop this issue. I was actually getting ready to pull the intake and replace it with a edelbrock one along with new valve cover gaskets to see if that woudl stop the leak. did you try double gasketing the distributer?