You've got to get the engine running sweet on just the carb before looking at the nos. Run some 8 plugs without the nos and tune the carb. You probably won't need colder than 9's even with the nos. You will want to retard about 7 degrees with that much nitrous. Rule of thumb is about 1.5* for every 50 hp nos. Whats your initial timing at, 32? 34?
I run a 632 with two speedtech systems. Initial timing is 32*. 1st stage nitrous is 400hp and I pull 11* Second stage is 300 hp and I pull another 5*. Thats a little on the sharp edge. Retard is recommended at 12* and 6*
I was running 10 plugs but now run 9's. They're easier to read and work fine. 10's will give you a rich reading when the car isn't rich... you lean it out and BANGGGG! If you can't read the plugs, pull off the headers and look inside at the manifold end. The colour there will help.
Read this article by Mike Canter. It is excellent. You will learn to read the plugs to tell if timing, mixture and plug heat range are correct.
http://www.dragstuff.com/techarticle...ark-plugs.html