custom resolution override - how to implement proper upscaling resolutions?

I’m using a custom resolution to be able to use a higher refreshrate (display has no scaler), thus the graphics part of my xorg.conf looks the following:

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "HYO DUAL-DVI"
    HorizSync       112.7 - 112.7
    VertRefresh     75.0
    ModeLine       "2560x1440_75.00" 396.79 2560 2760 3040 3520 1440 1441 1444 1503 -hsync +vsync
    Option         "DPMS"

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 1070"
    Option         "Coolbits" "12"

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "ModeValidation" "AllowNonEdidModes,NoEdidMaxPClkCheck,NoMaxPClkCheck"
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "DFP-0"
    Option         "SLI" "Off"
    Option         "MultiGPU" "Off"
    Option         "BaseMosaic" "off"
    Option         "UseEDIDFreqs" "false"
    SubSection     "Display"
        Depth       24

The 1440p 75Hz resolution works fine. However, upscaled resolutions don’t. For example, when I select 1080p in the Nvidia control panel, a part of the rendered picture is clipped. Or if some fullscreen application tries to select a lower resolution, it can even end in nothing but corruption.

So, could somebody please tell me if there is a solution, e.g. by adding the upscale resolutions manually and probably forbid the use of any other resolution? There are some 16:10 resolutions available which I don’t need.