Do you mean like the ones below?All you need are two flat backed washers instead of the curved washers that will fit onto the existing frame shifter mounts to get the shifters to fit. No drilling, cutting or hacking required. I can post up some photos later to explain if needed.
Shimano Dura-Ace SL-7800 Lever Boss Cover
For use with Shimano Dura-Ace SL-7800 10 speed shifters. This Lever Boss Cover can also be used on the following shifters: - Shimano Dura-Ace SL-BS77 - Shimano 600 Ultegra SL-BS64 - Shimano SL-BS50 - Shimano Dura-Ace SL-7900 - Shimano Dura-Ace SL-7700 - Shimano Dura-Ace SL-7402-8FCAI - Shimano...
www.sjscycles.co.uk
It does say they're compatible with 600 SL-6400 shifters, amongst others.