My wifes ggg grandfather was Lacy B. Castleberry. He married Elizabeth Parker the day after his sister, Mary Ann, married Martin Parker. Elizabeth, Martin, and I believe Lacy (the grave next to Elizbeth is unmarked) are all burried side by side at Tabenacle cemetery in Yalobusha county, MS. I beleive Mary Ann will be burried with the other family members in the Salem Baptist Church cemetery in Coosa.
By accident I found this website which you may find of interest:
There are many pictures, records, links and contacts. If you click on the Parkers, then again on Martin you will find his census records. I see William is at home in 1860, but not so in 1870 when he would be 19. Odd thing is, this site does not mention William in the genealogy, while his brothers are there in detail.
I did a quick check on Ancestry.com and found some William Parkers in Mississippi that are from AL and the right age, but I would need more to go on. I have a lot of information on this family. Please let me know what you are looking for.