Hi Twitter,
Welcome to Rootschat.
According to the monumental inscriptions for Glassford parish churchyard there are 2 stones that mention the name Sinclair and one stone with Menzies on it.
No sign of James Sinclair but I wonder if this could be his parents? It's a simple marker stone that reads - "The burying place of Archibald sinclair and Margaret Lowrie his wife and their children 1805". This stone would have been erected to mark the family plot (Plot owners were encouraged to do this by the kirk session because of "interlopers" taking over the ground) it does not mean that the entire family died in 1805.
The other stone belonged to a family by the name of Woodburn, "Elizabeth Sinclair died 27th July 1911 aged 70 years, wife of George woodburn". If you think this is a connection I will send you the full transcription.
The 3rd stone is interesting - I wonder if James' wife remarried after his death? Could this be her?
"Robert Crozier died 24th December 1898 aged 79 years. His wife Margaret Menzies died 8th December 1898 aged 80 years. Their daughters, Isabella died 17th March 1853 aged 7 years and Margaret died 29th February 1876 aged 22 years".The age is right, Glassford was, and still is, a tiny parish, and this is the only Menzies in the entire list of inscriptions.
Good luck,
Lodger.