How can fix HDMI HDTV overscan when I my TV has no aspect ratio setting?
You may be able to use xrandr to add a new mode to the specific output and then select that new mode. See xrandr --addmode and xrandr --mode in the xrandr manpage.
It would seem a bit weird for a TV to have 768 lines: HDTVs are usually 720 or 1080 lines, so if the computer thinks the screen is 720 lines I'd be inclined to think it is correct.
Is it possible that the TV itself has some options to scale the input? Check to see if there is a "full pixel" or "100%" option, or something similar that will let the display the image over the full screen.