David, Looking again at Samual Mckee's marriage of 1762 would suggest his birth about 1730-1735 and his father born about 1705-1715. This is very early for Scots-Irish in Ky-Va or even PA. Looking at my PA notes by Wm H.Enge, some unpublished, the earliest references are emigates out of Ulster or southern Scotland. The Scot-Irish migration began about 1719 but was in full swing by 1730.
I would suspect your Samuel's family came over from Ulster and his father could have accepted a Penn land grant in western PA, as many did, to buffer Indian raids. Samuel's marriage in 1762 could have occurred about the time he moved down from PA to a more peaceful place.
The Butler Area Public Library, 218 N. McKean St. Butler PA 16001 has good sources on early Scot-Irish in western PA and are very helpful. Good luck