Dave_You just have to mess with the monitor timings. I suggest CRU/using the nvidia control panel to do it. http://www.monitortests.com/cru-1.1.2.zip
This is what I use, I would just edit one of the Native Resolutions if I was you and change the Timing to LCD reduced and set it to 75Hz.
ArieYou'll want to check if your monitor even does 75Hz properly without frame skipping
http://www.testufo.com/#test=frameskipping use this if it says "Valid" at the bottom you're good.
As for the HDMI bit
HDMI has plenty of bandwidth for 144Hz at 1080p (HDMI 1.4 and 2.0) 75Hz at 768p should be not a problem at all, the real issue with HDMI is that it creates Input Lag (10-30ms approx.)
Dual Link DVI, High Quality VGA (such as cables that come attached to Diamondtron and Trinitron CRTs), and Display Port (also carries audio if that's what you use HDMI for but not every monitor has it) are the best cables to have for low input latency and high bandwidth.