You can tweak the image with the Advanced timing tweaker in Sampling opt: http://junkerhq.net/xrgb/index.php?title=OSSC#Advanced_timing_tweaker
H.backporch & V.backporch affect horizontal/vertical position. H.active & V.active affect size/cropping.
However you should update to firmware 0.81 which added a 640×400 preset that should already have the correct settings.
BTW the black looks crushed in the test pattern. You should select Full Range at Video recording device settings and try other video formats like XRGB.