Weather you have 18 to 1 or not, it's very uncommon to run that high of compression and no I wouldn't advise running nitrous. but then again I'm not real familar with the sb2's and what they can handle, maybe a standard 150 plate system wouldn't be to much? or a small fogger but either way your gonna want to get a programable nitrous controller to control detonation in a motor that's already prone to detonation refering back to what you stated, so I'd say you would be flirting with disaster by adding NOS to begin with.
I do know sb2's are nascar based engines and that they do run some pretty high compression ratio's to get the hp numbers they do, but then again they run their rpm's much higher than what you stated you were running yours at, perhaps all you need is more gearing to tap into the upper rpm power that's already there?
goodluck, Charles