Script form is not difficult; it's tedious though. Actions that the actors do and emotions that you want them to show are typically written in italics. Spoken dialogue will generally be indented once. The script will then indicate who is speaking followed by a colon. Next, type out the words. For example:
Fortunato is slowly becoming more and more alert. He begins to frantically strain at his bonds. His eyes dart from location to location. He is looking for help in any form.
Montresor: (sneeringly) There is no one here to rescue you. There is no one around to hear your screams.
Fortunato: (pleading) Why are you doing this to me? Please let me go. I'll give you whatever you want.
That's the basic form for scripting. There are variations to it that I have seen, but nothing that deviates too greatly.
As for the new ending, that's totally up to you. Because Poe's ending has Montresor successfully murder Fortunato, perhaps your new ending could have Fortunato escape somehow. Either he breaks his bonds, or somebody comes down the stairs and sees what's happening. You could also have Fortunato still die, but have Montresor get caught a few years later. Then you could reveal that he is writing about his crime from a prison cell.