Dale,
Generally speaking, you don't really want to hook your NTSC monitor up directly to your computer card.
The whole POINT of using a professional NTSC monitor is to see what's actually going to tape. So most people hang their NTSC monitor off the outputs of their RECORD DECK.
I don't know about your particular computer configuration - whether you're running Firewire, some form of transcoded Analog IO, or even SDI, but the point is that you want your monitor looking at your signal flow AFTER all the transcoding and A/D stuff is done.
Your mastering deck is at the end of the chain. It will likely have a digital to analog translation function built in. So all that's required is a simple video connection between the deck output and your NTSC monitor's input and you're good to go.
Good luck with your setup.