Life magazine printed 30 frames from the Zapruder film a week later in the 29th of November 1963 issue and considering the time it took to get the film and then the time it takes to organise, write, format, print and then get the magazine distributed across the country left the amount of time the conspirators had to do any alteration to be a couple of days at most.
In the following gif the Life magazine photos have superimposed over the original Zapruder Film and all the way through the most important events we get a perfect match to every image which means that to add a new element across a number of frames then there will be a knock on effect to these surrounding frames. The only frames we don't see are the actual headshot and the resulting back and to the left but if you had the time wouldn't these frames be the ones you would alter? Doh!
So in conclusion these Life Magazine photos published a week later are all the proof anyone needs to show no Zapruder fakery.
But in addition to this powerful evidence there is;
perfect synchronization between the Muchmore/Nix and the Zapruder films
the impossible to fake ghost image in the sprocket area is an accurate representation of images captured with Zapruder's type of camera
and upon microscopic examination the Zapruder film shows the correct and overall consistent grain level for the relevant Kodak film stock.
JohnM