Widescreen letterbox upscale and vertical-only stretch on ACIV

So, I'm trying to play Assassin's Creed IV on my laptop with a widescreen monitor. I set details to lowest and began lowering the resolution untill I got comfortable with the framerate. Unfortunately, the comfortable resolution is non 16:9 aspect ratio, which resulted in letterboxing.

Now, the letterbox would normally be alright, and since my monitor is actually 16:9, the AC could simply stretch to screen and vertical upscale to hide the letter boxes, I could have the game fill the entire screen with no picture stretch - unfortunately, that doesn't happen.

The reason why I expected it to happen is because I am running Watch Dogs on a very comfortable framerate (surprising, right?), with the same resolution, and it does exactly that! It simply hides the letterboxes and fills the entire screen, albeit at a much lower resolution.

I am not a framerate junkie, nor a resolution junkie (as you might have guessed), but having a smooth experience in Watch Dogs, while the game fills the screen, got me used to smooth framerates.

Anyways, anyone got any idea how to make it work in ACIV?

Edit:

Here are the screenshots from the games (locked into the actual aspect ration of the set resolution): ACIV (see the letterbox?): W_D: