Hi, Asia - I suggest that you start with getting the death certificate from the State of Arkansas for your Wm. C. "Neill" Biggs - the death certificate states that his mother was Margaret Johnson.
Then, if that doesn't help to solve your questions (or you have more) then re-post.
And was wondering if you have on your family tree that his mother was Cherokee why you posted it on the Creek Nation board? Maybe you'd want to post on Clark Co., Arkansas, board too.
Here's the entry for Arkansas Dept of Health - the date of death is right so I bet it's yours:
First Name Last Name State of Birth County of Death Death Date Mother's Name
WILLIAM BIGGS GEORGIA CLARK 9/30/1944 MARGARETT JOHNSON
Lastly, I checked the 1860 slave schedule for Biggs - John F had four slaves but none match your Neill so he's not with that family at that time. And his dad, Amariah Biggs didn't have slaves in 1860.