Read this article by Mike Canter, he knows what he's talking about -
http://www.dragstuff.com/techarticle...ark-plugs.html
I run a 634ci with two speedtech foggers. Here's what Speedtech recommend re retards:
Horsepower Retard Degrees
200 6
250 7.5
300 9
350 10.5
400 12
450 13.5
500 15
Rotor phasing should be set at lowest timing the engine will see with all retards on.
Fuel pressures should always be set flowing with a .073" orifice.
I would run a 9 plug. I run Autolite 3932 (which are a 10) plugs or 3933 (which are a 9)The article above will tell you how to tell if too hot or cold. The gap should be between 22 and 25. It depends how good your MSD and coil is are. Some say too bigger gap can result in the spark getting 'blown out' by the nitrous and cylinder pressure.
My engine has a base of 32* advanced. I then bring in 400hp nitrous on launch with a 11* retard. Then about 1 second later the second stage hits with another 300 hp at 5* retard, taking out a total 16*. I have a Jesel belt drive distributor with mallory cap, and have set up the phasing in between the initial 32 and the remaining 16* so its about the 16* mark when firing. I put in a T shaped rotor to make sure the spark isn't burning the edge off it when the nos is on or off.
You didn't say what size your engine combo is so some things may differ a little. I wouldn't use a blower with that much compression.
I highly recommend you read that article.
Cheers
Wayne