With the 64-bit client files in place you will receive the error: Blizzard Updater failed with an internal error.
Simply delete the 64-bit files listed below from the World of Warcraft folder, and the patching process will resume after going through a short repair process.
World of Warcraft-64.app
A new version of the 64-bit client files will be available for 4.3.4 after the patch is live. Please keep an eye on the 64-bit client forum thread for updated files.