I dug this game out again recently, probably for the first time since I had a widescreen monitor, and couldn't find any other posts on here about it so thought I'd report what I found.
Your chosen settings are held in a file Program FilesGames - Secret Weapons Over NormandyGameDatasaveconfig.cfg
I played around with changing the resolution and examining what it changed in the config file, and from what I can tell byte 16 (hex) appears to hold the chosen X resolution divided by 8, and byta 1A appears to hold the chosen Y resolution divided by 8.
So I did manage to tweak the game from 1600x1200 up to 1920x1200 by:
1) In the game, setting the resolution to 1600x1200 and the various detail settings as you wish them.
2) Exit out of the game.
3) Look at the config.cfg in a hex editor, you'll see byte 16 contains C8 and byte 1A contains 96. C8, 96 hex = 200, 150 decimal, x8 = 1600x1200.
4) Change byte 16 from C8 to F0. This gives F0, 96 hex = 240, 150 decimal, x8 = 1920x1200.
Like so:
The HUD appears correctly (not stretched) and it appears to be Hor+, so it actually works really nicely.
1600x1200 (4:3)
1920x1200 (16:10)
Unfortunately 2560x1600 would require a horizontal hex value of 140... which doesn't fit in one byte. So I've not found any way to set it any higher than 1920x1200.
Implode.