The 6400 series Ultegra was indeed 8-speed and the derailleur should be compatible with the brifter. Is your crank a double or a triple as these brifters were specific.
Also, Shimano front derailleurs have a small tab right inboard of the cable clamp bolt. the cable must be routed OVER the top of that tab and then through the groove on top of the operating arm under the bolt. If you pass the cable between the tab and the bolt the arm movement won't be correct.
Last edited by HillRider; 01-09-14 at 12:37 PM.
Reason: To answer the correct question