I am not much qualified to give advice since I don't even have a monitor yet but, have you tried the following with Soft-15kHz:
Street Fighter II (CPS1/2): 392 x 240 @ 60Hz
Mortal Kombat: 401 x 256 @ 53Hz
The second one is supposed to be especially for MK
If nothing else works, you can try to add your own modeline in a textfile, call it custom15khz.txt, place it in the same directoy as 15KHz and press the install 15kHz button to include your new resolution.
Here is a tutorial on how to create your own modeline:
http://community.arcadeinfo.de/showthread.php?t=7921The modeline should like this example:
Modeline "320x240@60" 6,86 320 359 399 438 240 242 245 261 -hsync -vsync