Julio Cesar Chavez moves up to welterweight like he did with Pernell and challenges Thomas Hearns of 1981.
Hearns would have taken Julio's head off with a right hand, like he did Duran...exactly like he did Duran, maybe just as quicly too.
Chavez had a very good chin, but that doesn't matter against a murderous puncher like Hearns, who also had very other advantage imaginable. Remember Duran was lucky to see round two, I believe Chavez would be as well.
It likely doesn't make a difference but the Chavez that faced Whitaker at welterweight was in better shape than Duran was against Hearns at 154. A few guys did go some rounds against Hearns at welter, and Chavez does have a good chin and good defence, so if he fought defensively he'd have a decent chance of surviving. But likely he tries to get inside and gets nailed. Most likely scenario imo is something like a 5th or 6th round KO.