I am doing a kill curve in HEK 293 cells with G418(0-1000ug/mL) and they don't die. They were previously transfected and selected with hygromycin and are integrated with a plasmid has also kanamycin resistance gene.
If your cell line already has the Kan gene integrated and expressing (as it appears to be given the G418 resistance) then you won't be able to use it for selection again. Can you use a different marker for your selection perhaps by changing plasmids?
Just to be sure are you using HEK 293 or HEK 293T this would make a big difference as the second line is resistant to G418 as they carry a plasmid for SV40 thermosensitive antigen