Jamma harnesses are standard. If what you have is Jamma, it should work (and be labelled right). Some older cabs, pre 1986 or so, used edge connectors with different pinouts, so it would pay to double-check. Count the pins if you need to and try to figure out what each one is for, vs the jamma standard.
As for the video converter - what are you trying to do? That thing is pretty standard for taking a 15kHz arcade board signal and turn it into a VGA signal. So you're using a VGA monitor, not the cab's original 15kHz one?