For dispatcher certificate candidates, the differences in the ADX written are primarily dispatch-related maximum duty day limitations -where the dispatch candidate is asked questions of both max duty period for both a dispatcher and a pilot, the ATP candidate is asked only those questions for a pilot.
Other than that, there are no real differences between the ADX and ATP written - and the performance is not just based on the B727, but I was asked in the written questions on DC9 and B737 performance as well.
There is a decent program available for purchase/download at
http://www.faatest.com; I am not a part of that organization, but that is what I used, and I passed.
If you want a decent FAR 121 text, get the ASA Publication "FARs for Flight Crew", google ASA FAR-FC.