If it absolutely can't leak I use rope. It does take a little power away but you don;t get leaks.
Given you're road racing - what do you have for crankcase evacuation? Have you done a leakdown at all to make sure the ring seal is still good?
I have both valve covers ventilated into a cross over pipe with a couple of breathers on it. There is almost no air coming out of those vents, so I don't think it is a pressure issue. My engine builder was concerned that I am using an old crank (that used the rope) in a new block (that uses the new type seals). Here is a picture of the system.