Marlene,
I am going to have to wish you luck from here. I looked at the World Connect family trees. They seem to agree that the Isaac Massingill that was married to Elizabeth Jones was born in Greene County, TN, in 1823 to George Washington Massingill and Mary Ann Gann.
It gets confusing then. James Stephen was supposedly born in Iowa, Jan. 7, 1836. (That would fit with him marrying in 1857. The counties that the Massengales lived in in Missouri and Iowa are actually right along the border. Decatur County, Iowa, is a good one to look in.) That would make Isaac 13 yrs old when James was born. There is a lot of conflicting information, so it may give you a place to start.
"Many" of the Massengales in Nodaway County, MO, were tied together with Matthew JR. son of Matthew SR. being their common ancestor. Check the Nodaway County, Missouri, and Decatur County, Iowa, message boards. I seem to remember some information out there about Harriet Burnet/t/'s family as well.
Good luck!
K