Barrera is an all-time great, but I think Barrera-Pacquiao II would have a similar outcome to their first fight. Barrera has just as much wear and tear on him as Morales does, it's just that Barrera has been able to hide it longer because he was willing to convert to a technical, more defensive style, whereas Morales knows nothing but going straight ahead.
Barrera can still outbox someone of Juarez's caliber, but Juarez isn't the offensive force that Pacquiao is. Pacquiao blitzed Barrera the first time, and all that's happened since then is that Barrera has just gotten older and slower, and Pacquiao has developed more weapons.
Barrera would last longer than the 2+ rounds that Morales did, but I think Pacquiao wins by mid to late stoppage.