Boop/Rob,
Perhaps this is your man. John Ferguson died age 70 at Porter Lodge, Ardlinglass on 18 Apr 1861. He was married to Sarah McCallum. The informants are son John Ferguson and widow Sara Ferguson. The death is registered in Kilmorich, Argyll.
John and Sarah had a daughter Isabel baptized 10 Jul 1831 in Lochgoilhead and Kilmorich, Argyll. You will find the family in the 1841 census transcribed at ancestry.com as Sherquson. They live at Ardlinglass House East Lodge.
In 1851 the household appears to be split with John at Lochgoilhead and Kilmorich. I could not find Sarah or daughter Isabella. Sara reappears in 1861 and with John resides at Ardno in Kilmorich, Argyll. Their nest is empty except for their grandson John McKean or McShean, age 4, born in Glasgow.
John McKean born about 1857 appears to drop off the radar about same time your John Ferguson born about 1857 appears. I wonder if John and Sarah adopted John McKean and changed his name to Ferguson.
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Colin