Clemens to Puzo to Blount: A triple-play of American letters

Who knew that one of the classic lines in "The Godfather" was cribbed from Mark Twain?

In an essay from the early 1980s, Roy Blount, Jr., describes touring Mark Twain's "High Victorian Gothic dream house" at 351 Farmington Avenue, in Hartford, Connecticut:

Nineteen mostly crepuscular but spirited rooms, kaleidoscopically decorated by Louis Tiffany. And a ground floor gallery where I viewed, among other mementos, a slate on which Twain would jot notes to himself. The slate, I am pleased to report, was not left clean. Few of the overlapping scribbles are decipherable, but I did make out two sensible reminders:

Leave the cat here.
Take the whiskey along.

Roy Blount, Jr.
Not Exactly What
I Had in Mind