I'd contact the Northwest Missouri Genealogical Society, http://nwmogenealogy.com/
. William isn't listed in their on-line list of obituaries but they might have more that aren't lited online.
They also might have a different index for this cemetery or other cemeteries in the county.
When William died, his wife might have been ill or didn't have the money to buy him a headstone. I also wonder if the was buried in the State Hospital's cemetery. Successful Searching!