Well I've never run that kit so I don't even know what hp jets it comes with, but for the most part a 75hp shot is safe with a stock car, a 100 hp shot requires the timing to be retarded, and colder plugs, and above that you might want to look into a better fuel system. I'm not sure if the Zex kit is a wet or dry kit, but if it's a wet kit then you can probably get away with even higher hp levels. I've known guys who ran 200 shot wet kits on 5.0L Mustangs all day long without problems....its a pretty big gamble and I wouldn't recommend it but it can be done.
As for whether nitrous is safe or not. Yes, its very safe as long as you follow all of the manufacturers recommendations. If they say to use a specific octane fuel, or spark plug go with it.
General Nitrous Oxide FAQ
BTW That car is a beauty and the picture of it at the track under the lights is excellent!