First bit of advice would be to never use the scanner's sharpening software, as that seems to sharpen the grain as much as the image. Sharpen later in Photoshop, where you can spot sharpen and also do it with a lighter touch.

Second, I have a feeling had the subject matter been anything but one that could be replicated today almost exactly, the grain would not have been noticed. I believe you got hit for it because when seeing 844 with UP cars, the screener probably didn't realize straight off that it was a 48 year old image.

Third, I wonder if the dark areas resulted in more grain being visible because you lightened them up with Photoshop? Was the actual slide darker than the image we are seeing here? Sure Kodachrome is pretty spectacular with grain, but if you pull the shadows way up, that kind of stress on the dynamic range will cause grain to appear, I believe (with my limited knowledge) because the lighter colors in there will vanish long before the darker colors reach a point that is acceptable to the eye. (Talking out of my hat here... not sure if this is exactly it... but the gist is, if you compensate for underexposure or pull up dark areas, grain will appear...)

Fourth, cool shot... thanks for sharing.