I went through the same O-ring deal with my hemi restoration because of leaks around the spark plug wire boots at the factory valve cover. I brought a tube into a specialty gasket/O-ring supply place. The guy was really helpful and gave me several ones to test out and found that if I used a #318 sized ring that it makes a good tight fit around the tube and a firm fit into the valve cover - You have to install it on the tube first then firmly push it into the valve cover. It is a bit snug getting it out but not that bad considering I have no more leaks. I do not know the specs of the O-ring that Arruzza sells. I also came across a metric sized one that fits really good as well. If you go the Arruzza route please let us know what size they are using. O-rings at the local supply house were about 45 cents each. At that price I put new ones in every time I pull a tube.

Good Luck

Jim