It uses the information that came out since the 1970s. The Congressional Investigation in 1977 resulted in a number of books afterwards.

If you read Curt Gentry's "J. Edgar Hoover, The Man and the Myth" you would know that Hoover had been filing false expense reports for decades, and built up a small fortune. (He stayed at hotels and ate at restaurants for free, then collected expenses as if he had paid.) If he ever left office, he would have been convicted for fraud, and died in jail. He had no other option but to die in office, since he could not (or would not) get a "get out of jail free" card. (The Watergate Burglary came apart when one of the burglars did not get this, as promised.)

You should know that this GOOJF card is not just an invented scene in the movie "Clear and Present Danger". Back in the 1940s the four-star General who headed the CIA went to President Truman with a complaint about an assignment: it clearly crossed the line into a felony. But Harry just wrote out a signed but undated full Presidential Pardon! Read the biography of Allen Dulles, "Gentleman Spy" for more details.

And LBJ's crony was implicated in various frauds, some of which were said to be in complicity with organized crime. LBJ was likely to be dumped as vice-president, and would also face prosecution, disgrace, and jail.

Mark North has collected a number of letters that passed between LBJ and JEH. Surely no one would expect either of them to put their plans in writing?