In East Alabama, where I came from in the middle of the last century, fixing to was not entirely synonymous with going to: it signified about to, on the verge of and would not be used of long-term plans or expectations. Thus:
She's going to be a pediatrician when she grows up, AND
She's fixing to be a pediatrician after she gets her degree,BUT NOT
✲She's fixing to be a doctor when she grows up.
And in fact the story you reference says that “the church was already reserved [...] and the minister all lined up, and the invitations sent.” Likewise, a little further on, the two-by-fours with which Sean busts up the car and Roxanne busts up Sean come from “a big pile of treated lumber that they got on sale from Wal-Mart” because “they were fixing to build a deck with” them and “Sean's brother was going to help them build the deck”. In both instances plans were pretty well advanced.
✲ marks an utterance as unacceptable