idle on an LT1 is really annoying to deal with, you need to get most of your air through the idle passages in your plenum rather than your throttle plates. they run best when you keep the plates totally shut and the IAC counts up. a 383 usually needs the idle air hole in the throttle body enlarged a bit. use eehack to mess with your timing advance at idle to see what it likes too. also don't be afraid to just increase your idle speed a bit until it behaves...