There is no stop (at least in the LS1 T56). If you're at 6800 RPM in third (or whatever gear) and accidentally pull the shifter into second (or the next lowest gear), you will hear a VERY loud bang, the tires will lockup, the engine will slam off the limiter, and if you're unlucky a lot of parts will break inside of your transmission and in your rear, not to mention you might bend pushrods, etc in the engine. I don't recommend doing this.
Not trying to scare you, but it's the reality. Get very comfortable with your car and build up the speed of your shifts (especially if power shifting) until you're where you want to be. Practice as much as possible.