I haven't found tombstone records for either William or John McGaugh and don‘t believe they probably exist (but I would love it if somebody finds our found it). I've looked extensively in Ray County Missouri for John McGaugh (and they seem to be very well indexed in Ray County). I'm fairly confident there is not a readable tombstone in Ray County for John; unless it isn‘t at an established cemetery. There are two McGaugh cemeteries in Ray County one is also known as McGaugh/Zirjacks Cemetery. I would guess he's in one of those but I've never found a listing at either. Two that I've searched on a visit to no avail is Sanderson and Crowley cemeteries; there are other McGaughs in those cemeteries. At findagrave's web site there are a fair amount of picture of other McGaugh tombstones in Ray County include Zirjacks. John’s will was administered in Ray County, in addition to that being where all his property was at the time of his death, I'd be fairly confident he's there in the County somewhere - but I think its either unmarked or the tombstone is too worn to read. As I think you mentioned I would hazard to guess the McGaugh/Zirjacks cemetery.
In the “History of Ray County”, from 1881 by the St. Louis Historical Society, page 248 lists John McGaugh as a Justice of the County Court from February 1823 - February 1825 and then again from September 1825 to February 1826.
As far as William, I haven't searched him as much, but from Larry James research in the late 1970s early 80's he indicated that William died in 1789 in what is now Davidson County, Tennessee. He had land there that was divided amongst his kids. Apparently the land was granted to him by North Carolina and was 20 miles south of Nashville. Larry James believed that William died in Hickman Station which is located near the community of Scottsboro. Larry also indicated he found deed records for William in Davidson county under the name of William McGuaock.
As far as spelling, most of the records I've found for John McGaugh on down has been McGaugh with an occassional McGaw (in the cases I've seen that I think is probably spelling error of census taker or the result of illiteracy as its far less common).
Hopefully this helps. What line do you descend from after John McGaugh? I come from his son John A. McGaugh to Daniel Thomas McGaugh.