Why was Montresor so proud of his plan?

Montresor is so proud of his plan because it was successful. He states in the last line of the story that for fifty years Fortunato has been walled up. Nobody has disturbed his final resting place.
Montresor must have conceived his revenge against Fortunato as a young man. He had endured, he says, a thousand insults and an injury. After the injury, he planned revenge. To him, revenge needed to be successful in two ways. First, the revenge had to go undetected by anyone other than the victim. Second, the victim had to know he was being punished and by whom. As Montresor puts it:

I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.

Montresor succeeded in both goals. His crime went for fifty years without anyone being the wiser. Fortunato also knew in the end that it was Montresor walling him up to die.