Interesting article in this morning's Washington Times, Witness to a murder, by Hugh Aynesworth, who is now the Washington Times's Dallas bureau chief, and in 1963 was a reporter for the Dallas Morning News, has the following two paragraphs:
The Metro section [of the Dallas Morning News] carried an interview with former Vice President Richard Nixon, in town under his lawyer's hat for meetings with Pepsi-Cola bottlers.
Nixon was scheduled to fly out of Love Field two hours before the man who barely edged him for the presidency in 1960 would land aboard Air Force One. At a Baker Hotel press conference, Nixon predicted his old rival might drop Vice President Lyndon Johnson from the ticket in his 1964 re-election campaign if the Texan proved to be a political liability.
Not only does this seem to settle the matter of Nixon's whereabouts at the time, it could also show that Nixon was reminding LBJ of why he ought to continue to go along with the conspiracy, if there was one.