When they say SYNC is negative, what is really meant is that it is negative as compared to the black level of the video. So if sync is at 0 volts, you just have to insure that what you want to be black is at about 0.4 volts and what you want to be white is at 1.0 volts. Most all video monitors capacitively couple to the video input, and do some sort of internal clamping to put the black level where they want it to be.