I think it would be particularly hard since nothing during the period of the Spanish Empire prepared any of the people to think in the same terms as Americans did preceeding their independence. They did not consider themselves united and they certainly wasn't, no matter how hard Bolivar tried. Problems with efficient communication would also pose great problems to maintaining any form of unification.
Leaving off Brazil, which I doubt would leap into unification with the Spanish portions, all it would take would be one civil war to fracture the Southern Union. Too many of the immediate successors to Spain thought locally rather than continentally.