After looking at the entry for Bosque County I am not sure this Wm is the same as the Wm in Cass county. Like you said the other names don't match up. Now it is possible for a person's family to be wiped out by an epidemic or an Indian raid (and I did have one ancestor whose entire family died in 1855-56 and he was raised by an Uncle & Aunt). There is another William Allen living in Rusk County who might be a possibility but again, the other names are not the same as the ones in Bosque County. He has four daughters: Eudora 16, Kisiah 8, Frances 6, and Margaret 4. Sometimes people just got left off the census. Some people changed their names. You never know! To get a pension record, go to the link I gave you and look up your person and it will give you instructions about how to get a copy of the pension application.