Let me see if I got it correctly: your screen is 320x240 and windows is oriented to a 320 width, 240 height, so it's actually sideways. In this situation you should launch attract mode, which will think the screen is 320 wide, 240 tall, and then toggle screen orientation (with a hotkey or options from the theme). That should tell the theme that the screen is now 240 wide, 320 tall, but if the theme is not programmed accordingly it will not switch to a "vertical" layout. Of course a vertical theme should work ok in that case.
If you want you can try my Arcadeflow theme and toggle orientation, and let's see what happens...