K-Lad,
The LDS shows a John Cowan b:ca 1758 Co. Down. d:9 Dec 1841 in Cowansville, PA. Son of John Cowan and Jane McCulloch. John Cowan (father) was born ca 1728 Co. Down died: 8 Dec 1811 Sugar Creek, PA.of parents from Lankarshire (sic), Scotland. John Cowan married Jane McCulloch in Lank. Scotland, where she was born ca 1731.
This may be your family as it is the closest that I can find based on the info in your original post.
Slainte