italian mafia in italy became a multi billion dollar powerhouse only in the1970s, because of the management of public works, gambling, international drug trafficking etc.
there are several italian bosses who are millionaire, or even billionaire, matteo messina denaro is really billionaire, police seized assets worth 1,5 billion euros to him, and that's only the part seized

i think that american mafia in the 19401960 period was the strongest worldwide, it depends what you mean for information
at that time there were by far less powerful criminal groups than today, italian mafia in italy, yakuza and triads were weaker than american mob i bet, yakuza was at its peak in 1980s (american mob in 1960s), italian mafia in italy was at its peak in 1980s too, i don't know Triads, but probably in the 1970s were at their peak
mexican and colombian cartels were nothing at that time, russian mafia didn't exist, so i think italo-american mob was really the strongest at that time