I tried that resolution but everything is still off. The loading screen and main menu all appear in the top left corner and the actual GUI is goofy looking as well.
I gave up on the Widescreen GUI mod. It is terrible. Just terrible. Sometimes works, sometimes not at all. If it works, the GUI will look screwed up, unless one installs another mod that screws up BG even more.
So, I gave up on that jazz and just set the GPU to keep the aspect ratio of 4/3 resolutions. So it runs as it was designed, no stretching on a widescreen monitor. It looks right to me.
Some games do not benefit from a higher resolution. BG1 is one of those games IMO, so to fix the aspect ratio is really the best solution.
For AMD users, go to Catalyst Control Center > Digital Flat Panels and choose Properties. Select "Enable GPU Scaling".
Then go to Desktop Management and pick a 4/3 resolution, such as 640x480 and press *Apply*. Go back to Digital Flat Panel and Properties, there choose "Maintain Aspect Ratio" and press *Apply*.
That's it you can go back to your native monitor resolution, and every time you run a game in 640x480 or 800x600 etc it will maintain aspect ratio automatically. Just fire and forget.