Your best bet would probably be a Rebel colony in Cuba. Spain in the mid to late 1800s was notoriously unstable, and the locals would at least be somewhat sympathetic to their racial ideals and would appreciate the addition of organized military elements willing to defend the status quo from the potentially uppity black population. Assuming you could get the US to not make a move on the island, a sufficent breakdown in Spainish power could easily lead to the Dixie exiles merged with the local elite making a bid for independence or autonomy within the Empire and winning it