Why does Montresor wait fifty years to tell his story? How might the story be different if he had told it the morning after the murder?

Montresor, like most of Poe's protagonists, is paranoid of being found out. The crime would have been too fresh had he told it the morning after. By telling it 50 years later, most people who remember Fortunato would have forgotten or have died.

Montresor is raked with guilt, which causes him to tell.  Montresor is haunted by Fortunato’s last words “For the Love of God, Montresor!” which is not a cry for mercy, but a threat against his soul.  Fortunato speaks of Montresor's own demise and the afterlife (God's wrath).  And many critics believe that Montresor is on his death bed, which likely means he is thinking heavily about the afterlife