Pac would cause problems and definitely have his moments, but Chavez would gradually break him down for a late stoppage imo.
Chavez on sheer durability. I expect Manny to look better for most of the bout, and would look for JCC to stop Pac late.
Because Chavez is durable, and Manny can be hit and knocked out, Chavez wins this one. Chavez was a battering ram, the best lightweight since Duran. We haven't seen one as good since.
I'm still not sure that Chavez is human. Whitaker fight was something what will never happen again(for good).