I have a bike that came with the back end out of alignment and the vertical dropouts too open as if too large an axle was inserted. After straightening the rear, I clamped the tops of the dropouts in a bench vice and gently closed the dropout slots with a deadblow hammer.

In this case, it might be possible to file the top of the opening on the low dropout, and then raise the bottom by bending followed by filing it back to parallel. I could see that buying you a mm or two.