Wallace on the ticket, especially under Humphrey would be ASB. Particularly assuming that the '64 Civil Rights act is passed as OTL. It would be a complete 180 on the Kennedy legacy on civil rights and something that a strong civil rights advocate like Humphrey could never stomach.
Gore would be more likely, if the Southern vote is absolutely needed, but only marginally more so. He was a key member of the filibuster of Humphrey's '64 bill IOTL. If the GOP goes with Goldwater as OTL, then such strategy would not be necessary, and Humphrey immediately dismisses consideration of anybody that doesn't excite him. (Similar to OTL when Johnson realized that he could dismiss RFK from his VP choices.)
Arthur Schlessinger wanted a RFK/Humphrey or Humphrey/RFK ticket as early as Nov. 23, 1963 OTL. As for the likelihood in a scenario where Humphrey is free to choose among the field, I'm inclined to say that RFK still ends up running for Senate and looking at '68 or '72. That plan was largely in place before JFK's assassination.
There was a little bit of bad blood and religious unpleasantness from '60 between Humphrey and the Kennedy family, though RB would better assess the personal relationship between Humphrey and RFK as of '64. They were indeed like minded on a fair share of the issues, though, at least at this point.
If Humphrey is looking for youth as an RFK-alternative, there were some others from the WWII generation just coming into play that year. Reynolds of WI, or Brewster in MD were young favorites who OTL were establishment loyalists who helped Johnson cement a '64 primary victory over Wallace.