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.
https://www.sjscycles.co.uk/gear-spares/shimano-sl6028-lever-boss-cover-for-alloy-frames/
It does say they're compatible with 600 SL-6400 shifters, amongst others.