This is the original script with Brando as the older Vito and Clemenza (who wasn't replaced by Pentangeli yet).

http://www.dailyscript.com/scripts/godfather2.html

I'm not entirely sure, but this is I believe the official final draft.

But nevertheless Coppola had to change the scenes with Brando and Castellano at the last minute.

"It was between the brothers Kay -- I had nothing to do with it."