For GOG version both Widescreen Fixer and Flawless Widescreen are not working. At least for v.3.4.0. W.Fixer sometimes accidentally works, but generally you can't rely on it.
For Steam version Flawless Widescreen is working no problem.
So it is either hex edit provided by someone else above or get ...
But with AC: Brotherhood the game worked with .exe fix, the game seem to be installed in different way than AC II, maybe III, IV and Rogue will work the same.
Played through on 3440x1440, everything is perfect except for shadows flickering in some places and super small texts, not scaling to well with 1440p, but in general very playable at 21:9 and looks very impressive for its age, still the best looking h'n's on the market. Also very worth playing as a ...
I have one question regarding 21:9 fixes for Assassin's Creed series on Uplay. I tried a fix for AC II but it wasn't working, saying that "UplayGameLauncher cannot be found, please reinstall the game" or something like that. So far this is not an issue for me because I got first AC from GOG, so ...