I believe that the following is the same land grant you write of:
DAVID KINKEAD, 660a. NL. St. Marks Par., Spolsylvania Co.. in the Goard Line Fork; mouth of abr. of Kinkcad Run to Bessie Bell Mountain; Mary Gray Mountain. 1 Aug 1734.
This one might be another land grant?:
DAVID KINCADE. of St. Mark's Par.. Spotsylvania Co.. 400a. NL. in co. afsd.. in great fork of Rappahannock R.; N. side of the head of the branch of Devil's run. 20 Jun. 1733.
St. Mark's Parish:
Formed from St. George's Parish. When Orange Co was formed from Spotsylvania Co in 1734, St. Mark's Parish went too. After Culpeper Co was formed from Orange Co in 1748, it served both counties. In 1753, that part of St. Mark's Parish in Orange Co, was added to St. Thomas's Parish.
Finally!! I found Devil's Run in Culpepper Co., VA. It intersects with Hazel River. I backtraced Hazel River and it begins off the Rappahannock River. It also joins up with Thorton River which is in the vicinity of Bessie Bell Mountain. Hazel River pretty much stays in Culpepper Co. This looks like a prime possibility for where these Kincade land grants were. I'm pasting the link for the map so you can see Hazel River and Devil's Run. Devil's Run would be to the lower right of the map. I'm sure if you play around with it you can figure it out.
Here's the link:http://www.mapquest.com/?q=38.53694,-78.11194
Hope this helps.