if that replaces the rear shock ie the weight of the vehicle now goes thru the shock mount- might be possible with strengthening, i havnt seen it done on your sort of car only lighter stuff like starlets, corollas etc.
IMO from what i can see in the pic they look like a bit of a cockup anyway, that spring would coil bind at that adjustment height, looks like lowering the spring platform would result in it being uncaptive so you would need keeper springs, cant see any bumpstops unless they are up in the mount tube thing but then they wouldnt be doing anything.
i dont rate bilsteins much either, i have found a few issues with them