If it goes blank then it sounds like either your TV or video card doesn't support that res. What are you running?
In answer to your question though, you can try both SwitchResX or DisplayConfigX, both just hack the display preferences to add custom ratios etc to the mix but be warned, these products aren't renowned for their user-friendliness.. it'll take a lot of trial and error before you get anything sharp/usable and you'll probably want to read up on timing and porch ratios before you start.
Hope this helps