Yes. Only available ARs in the display options are 5:4, 4:3, 16:10, 16:9. By default 1280x768 is 16:9 stretched, but you can make that 16:10 squashed...
To elaborate on my previous reply, I couln't find a way to achieve Hor+ in widescreen with the three variables in main.ini : PixelAspect, Aspect, Resolution. All I can do is change the vertical FOV. I guess the hor FOV in widescreen is tied to each "stock AR", which would explain why the FOV in TripleHead is basically a multiple of a "stock FOV".
3840x1024, Aspect 15:4 : same hor FOV as 5040, Aspect 48:10
Oddly though, on my very first run in Vista (30") the game seems to have used my XP desktop's AR (WTH) to calculate the FOV somehow, so I played a bit in a slightly distorted, Hor+/Vert+ 1024x768. I couldn't reproduce this with main.ini though ! (I used to play at 2560x1600 in Vista and 5040 in XP on the same PC until yesterday. I installed the game in XP only.)
1024x768, same hor FOV as WTH again - but how did I do that ??