I believe he is the William living at Gauly Bridge, Va(W VA Now) where you will also find James Britt Muncey whom I believe to be his brother or 1st cousin.
A William Muncey was the administrator of the estate of Luke Muncey Sr in Montgomery cty, Va circa 1831. This may be the guy.
Remember that it was common in this family to continue naming children after kin so it may be hard to prove.
Luke Sr's will did not identify William as a son. It ALSO did not name NATHANIEL CALVIN MUNCEY as a son either though I have proved that he was a son. It happens that sometimes fathers give thier sons thier inheritance in thier adulthood or upon need. Thus having received same, they are not listed as receiving from the estate.