Replacing Shazam! as the new feature, The Challengers of the Unknown took the lead spot in Adventure Comics, in a tale called "When Destiny Calls!" written by Bob Rozakis and drawn by George Tuska and Andy Mushynsky.
At first I thought these were inventory stories from some other book, but a comment from inker Andy Mushynsky tells me they were all-new, done especially for Adventure. Thanks Andy!
The other stories are:
Superboy in "Lana Lang and the Legion of Super-Heroes" by Otto Binder and George Papp
Aquaman in "Is This My Foe?" by Steve Skeates, Jim Aparo, and Dick Giordano
new feature Zatanna in "A Nightmare Called Gorgonus!" by Len Wein and Giordano
Supergirl in "Supergirl's Three Super-Girlfriends!" by Jerry Siegel and Jim Mooney
Captain Marvel is still here, but in reprinted tales, like this one from Captain Marvel Adventures, "Twisted Powers" by Binder, C.C.Beck, and Pete Costanza
The Spectre in "The Ghost of Ace Chance" by Gardner Fox and Murphy Anderson
...in addition to the Legion-centric text page by Paul Levitz, there's an additional one by editor Carl Gafford helpfully listing where all these stories have been reprinted from. Another nice cover by Keith Giffen and this time inked by Romeo Tanghal, too!