Exactly my point; for this type of precision shooting, a mount is a necessity. I believe the people behind the assassination knew they would never get a second chance. So they carried it out using multiple shooters using a classic military sniper style triangulated cross fire method. Once the car crossed into the kill zone, they let loose.

Craig Smith talks about this in much deeper detail in his book:

Kill Zone: A Sniper Looks at Dealey Plaza: Craig Roberts: 9780963906205: Amazon.com: Books