On line 14 of the index.php for a gallery, you can change the size of the image it uses. Using slideshow_dissolve for example, you'll see 782c in the url. Change that 782p to use the proportional image (c = cropped, p = proportional).
The slideshow_wide is so wide it is not really designed to work with a proportional image, there would be tons of extra space on that one. I would recommend pre-cropping your images for that particular gallery if the auto cropping is not looking how intended.