I've heard different numbers too. Some numbers seem to come from people with % of cars built with certain options and then they break it down by production engine,tranny, shaker, etc. After all these years of car production knowledge being learned and shared, if the exact # built of 71 Challengers with shaker were known, it would be more common knowledge. Needless to say, it's a low number.
Galen could tell you how many he knows of with shaker and 383, and how many without, and then you do the % to get a close approximate.
Good luck.
Tom