This site has it.
http://mcadams.posc.mu.edu/route.htm "As the date for the President's visit approached, the two Dallas newspapers carried several reports of his motorcade route. The selection of the Trade Mart as the possible site for the luncheon first appeared in the Dallas Times-Herald on November 15, 1963. The following day, the newspaper reported that the Presidential party "apparently will loop through the downtown area, probably on Main Street, en route from Dallas Love Field" on its way to the Trade Mart. On November 19, the Times-Herald afternoon paper detailed the precise route:
The motorcade will then pass through downtown on Harwood and then west on Main, turning back to Elm at Houston and then out Stemmons Freeway to the Trade Mart.
Also on November 19, the Morning News reported that the President's motorcade would travel from Love Field along specified streets, then "Harwood to Main, Main to Houston, Houston to Elm, Elm under the Triple Underpass to Stemmons Freeway, and on to the Trade Mart."
I just found out today that everyone knew about the route.
The reason they didn't go down Main is because the limo was too low to jump a curb to go from Main to the Stemmens Freeway.
Elm Street leads to the freeway.