I’ve only seen one Superscope movie, The Day The World Ended and it was obvious from my old fullscreen copy it was simply cropped in theaters. You can compare shots in the trailer to letterboxed stills.
https://www.youtube.com/watch?v=ELFA1fdNAbw
http://bmoviefreak67.blogspot.com/2013/03/day-world-ended-1955.html

The only scant evidence widescreen was planned for Fantasia in 1940 is a very murky shot of a mixing session, at the bottom of this page:
http://www.widescreenmuseum.com/sound/fantasound1.htm