From 1999-2002 they made an ACR (American club racer) edition Viper.
From 2008-2010 they made an ACR as well.
Both generations of ACR have adjustable suspensions, special track features etc so you can go and race at a track and have a bit more tune-ability for the growing racer. Both of these are also street legal.
The ACR-X is a stripped down 2010 ACR that has no emissions equipment, 640 hp, no radio, HVAC system, windows, sound deadner, etc and is no where close to street legal. Also as there is no VIN number (just a Chrysler chassis number for identification purposes) there is no way of making it street legal even if you wanted to.
Hope that helps!
Robert