My initial thought was to pick Napoles but, I dunno. Napoles never fought anyone like Whitaker, and Pernell looked phenomenal at 140 against Pineda. I think this is damn near 50-50.
I think that Napoles would win. He isn't going to let Whitaker set the terms, Napoles would be pushing all the time and making Whitaker have to react to him. Whitaker, like just about every top fighter, is used to fighting on his terms so he is going to contest the issue. But when they clash, I don't see Whitaker winning too many exchanges. To win the fight Whitaker has to make it a game where he only fights when he wants to because he can't outfight Napoles and Napoles will be pushing him all the time.
One thing to consider is that southpaws were still a very rare commodity even in Napoles’ time. They were still primarily converting lefties. I don’t think they became widely accepted until the late 70s and particularly the 80s. Napoles only faced one (I think) in almost 90 career fights.