I am building a 12:1 LT1...

What octane would it take to run a 175-200 shot on it?

 

The fact that you are at 12:1 doesn't mean that much. It's your dynamic compression ratio that matters. Do a search, there was a really good thread going on Advanced Tech a couple of weeks ago.

With that size of nitrous shot, it wouldn't hurt to dump in some 100 octane unleaded.