Looking at the movie it's obvious that while the lookalike model was very large, it was definitely not at one-to-one scale. The Rushmore Presidential eyes alone are 5.5 meters wide. Plus when you're at the Mount Rushmore National Monument in South Dakota it turns out that Hitchcock’s set was not so much a lookalike as a look-the-other-way. The orientation of the heads (particularly Lincoln) appears to have been tweaked to get the shots Hitchcock wanted for his chase sequence. Of course clambering around the 18 metre high faces without specialist equipment is impossible so for his set Boyle added flat surfaces, hand holds and, down the side of one face, all but a set of stairs.
In fact Hitchcock nearly didn’t get to build his Mount Rushmore at all. It was only by making a few promises about not having any violent scenes associated with the monument (whoops that didn’t happen) and not showing the faces above the mouths (that didn’t either) that he got started and then he followed the old rule - don't ask for permission, beg for forgiveness.
You can see the Mount Rushmore sequence from North by Northwest here
Images: Mount Rushmore last week and ‘Mount Rushmore’ 1958
