How would I go about fitting a window AC into a really wide window?
That is a really wide window.
At just 12 inches tall you won't be able to fit any sort of window unit. Even 6,000 btu models are usually 12.5 inches tall. If you can find a short enough model then great but then you'll have to contend with finding a wide enough accordion curtain or just fill the void with cardboard.
An alternative option is a portable A/C which sits on the floor and exhausts out the window using a vent tube.