You might want to add Blair Drummond to your list of places - it was Henry Home, Lord Kames of Blair Drummond who had the moss (which was part of his estate) drained in the late 1700's.
Blair Drummond is now-a-days mostly known for it's Safari Park, but also has the 'ancestral pile' right in the middle of the estate.
On part of the estate, there is a tiny old churchyard (sometimes known as Kincardine-on-Menteith as well as Blair Drummond) and there are (or were) a few McGowan's listed on gravestones there. The MI's are contained in 'Pre-1855 Monumental Inscriptions, South Perthshire'.
I have a copy of the above publication and there are 4 McGowan MI's listed:
1.
George McGowan, late farmer in Coldoch, died 24th October 1795 age 66 and on the same stone,
George McGowan, writer in Stirling, died 8th May 1836 age 35
(probably his son).2.
1857
George McGowan, late farmer in Balenton died 9th July 1845 age 80 and his wife
Janet Finlayson died 25th October 1845 age 80.
3.
Robert McGowan died 23rd December 1846 age 85 and his wife
Margaret McKinlay died 19th March 1837 age 73.
4.
R. M. I. (or J.) D. 1808
Robert McGowan, late tenant Coldoch died 23rd August 1806 age 51
His widow
Jean Dawson died 26th March, interred 30th March 1840
Jane McGowan or
Dow died 1st January 1881 age 78
(probably their daughter, married name Dow)I know these are all after you ancestors left Scotland, but maybe some of these are the ones left behind?
Anne