49-450A/B Is this a Radio Shack product?
I found some data on that console.
Yes your problem is the NC switches are being used to trigger the 49-450 and its sensing circuits are messing you up.
Do you have a siren on it? It has a siren output and it switches 12 volts to the siren. You can use an ELK912 Security Relay to trigger the SC1200. Just wire the ELK912 + terminal to the + Siren Output and the ELK912 - terminal to the - Siren Output. Then use the NC contacts on the ELK912 connected to the zone 31 or zone 32 of the SC1200.
Only reason I recommend the ELK912 is it has a nice terminal block for connections and a protective diode on the relay coil to suppress noise when switching On and Off. If you are handy any 12 volt DC relay would work if it has a NC set of contacts, but a diode across the coil would still be a good thing to do.
I would strongly suggest you do NOT leave the SC1200 on the alarms NC zone terminals. They are not outputs they are inputs. Its sensing voltage may damage the SC1200 or the 49-450. Use a relay or something to get you an isolated output to the SC1200.