The 1850 census for Joseph McClanahan's family was taken on October 28th. With the marriage performed on October 22, the Hannah shown in the household could be your married Hannah Miller with the birthplaces transposed. It would definitely be worth looking into.
Some of Stewart County's probate records are available at familysearch.org, but I didn't see any records for Joseph. The courthouse would probably have a master index of some sort, and I see that the LDS Church has microfilmed a good bit as well.
If it were me, I'd check that county's deed records to see if you can spot the heirs of Joseph selling off his land. William may turn up on such a record. Online family trees show that he died in 1862.
Here is a link to the county's archive, it may be easier to get copies of these documents from them directly...
http://www.tngenweb.org/stewart/archives.htmYou might also check to see if the original marriage license is available for William and Hannah. Sometimes the bondsmen and witnesses shown were relatives and may offer additional clues. The attached copy is from ancestry.com, so I'm not sure what else is available from the courthouse.
Good luck, hope this helps,
Scott