As of 1967, 5,000 people were involved in organized crime. In the 1980 census, there were 12 million Italian Americans living in the US, so let's assume there are 10 million Italian Americans in 1967.

Assuming everyone in organized crime is Italian American, at its peak 0.05% of Italian Americans were in organized crime.

edit: Yes, I know it's unrealistic to assume everyone in organized crime is Italian American, but my main point is that a very small amount of Italian Americans were involved in it. We will never know the actual statistics because people generally don't put down on the census that they are a criminal in organized crime.