Why do you want to cut of one third of the picture?
Maybe because it's just black "unused" screen?
It's not "unused". It really is
There is no information there; it's the same issue you had with 16:9 or wider aspect ratios on the old 4:3 TV-screens.
You can only fill your screen by either losing actual picture information on the top and bottom ends or by stretching the picture. Neither will improve the picture quality.