cropped images: force aspect ratio for small ones

I created an image size 800×450 with cropping on. It works fine with big pictures. But sometimes users on my site have to upload smaller pictures. For example 700×700. This kind of image is cropped to 700×450 and aspect ratio 16:9 is gone. How can I force wordpress to crop this picture into 700×394?

Or if that’s impossible should I find a plugin that stretches images to a minimal accepted size on upload? Does a plugin like this exist? I know those pictures will be fuzzy and generally it’s a bad solution.