Clemenza was the highest paid actor in I. Asked for too much in II, so they canned him.
Hyman Roth was based on Meyer Lanksky.... Russian Jew mobster who financed Italian Mafioso. Bugsy Siegel is Moe Green. 80-90% of the movie was barely re-written truth based on well-known Mafioso. The Cuban scene happened; cut real short in the movie. Great Havana casino was built and thrived with Hollywood performers for a short bit. Sinatra et al were in on it. That's where all those beautiful old 50s cars came from. Cuba was incredibly ritzy for a year; lawless city of decadence.
Castro came in with his uprising; seized it all, banned gambling...etc.
I've even heard that the part where "Superman" performed at the XXX club (where Michael learns of Fredo's betrayal) was based upon reality.