Check the man page for the xorg.conf file, as well as the readme for the ATI driver, and see if maybe a 'virtual screen' size of 1366 x 768 would provide the output you want.
That's an odd size; if I may, why does it need to be that 'exact' size? Much of the time (like in my case here at home too) the TV will auto-configure its screen size; I can send it anything from 640x480 up to 1024x768, and the TV sticks it on the screen the same way regardless, they all look the same.
EDIT: Thankz Archtoad for pointing out that typo
which has been corrected.