Ahhhh, it's nice and easy with NVidia!
I no longer have my gtx 670 so this is all from memory:
Open up NVidia control panel (right click on desktop etc)
On the left hand side you should see 'Display', click on that to show a bunch of other sub menus. The one you want is 'Change Resolution'. Within this menu you should have a button 'Customise' at the bottom underneath all the resolutions. Click that and then in the popup click 'Create Custom Resolution'. Also make sure that the tick box above this button 'Enable resolutions not supported by the display' is checked.
Now you need to type in the resolution so: Horizontal 2560 Vertical 1080 Refresh Rate 75 Colour Depth 32 Scan Type Progressive.
Now I cant remember with regards to 'Timings'. You don't need to put anything specific just something along the lines of Auto or something like that.
Next click Test. Hopefully you should still be able to see everything. A box pops up asking if it worked, if so click yes and it saves the resolution. if your screen goes blank then it didn't work, just hit escape and that should bring things back to as before. Now, the resolution should appear within the first popup that arises when you click 'Customise'. Make sure that the box is ticked (it might not be) otherwise it won't show up as an option within the NVidia control panel.
It SHOULD work given that this display tends to even allow above 75hz but it skips frames so is pointless. 72hz is a good one for blurays btw.
Hopefully that helps, it's been a while since I used a Nvidia card
