Roger, Are you sure your grandfather is buried in Bell County or are you assuming 'since he lived there he is buried there'?The reason I ask is, Trannie C. McWilliams died in Williamson County, Texas on 8 April 1964.It also indicates he was a widower.
In all probability he is buried next to his wife who passed prior to 1964 so if you know where she was buried, you can perhaps find his grave rather easily.This is not a hard and fast rule so you may also find he has been buried in Williamson County.
The TxGenWeb Bell County web site ( http://www.rootsweb.com/~txbell/http://www.rootsweb.com/~txbell/ ) has many of is numerous cemeteries cataloged.I checked those near Holland that were listed but failed to locate his name.If you know the name of the cemetery where your grandmother is buried, and it is in Bell County, you might find their names on the list for that cemetery.If your grandfather was married more than once then he becomes a bit harder to locate.
You can also obtain a copy of his death certificate and while there are no guanantees, his place of burial may be shown.You can go to http://www.vitalrec.com/tx.htmlhttp://www.vitalrec.com/tx.html for assistance in obtaining a death certificate.
You may also try to obtain a copy of his obituary which generally indicates where burial was performed.The largest library in Bell County would be the place to start looking.
I hope you are successful in locating the grave site.
As far as the other grandparents, I could only locate a death record for Nellie L. Hooten who died in Lamar County 25 Jan. 1994.I have no way to determine if she is your grandmother.