Remember, Hearns was 6′ 1″ and Manny is like 5'6'". Hearns is not Antonio Margartio although he punch like he had loaded gloves. Hearns was fast and had a hell of a jab. Now I remember he out jabbed Sugar Ray Leonard. Manny would put up a hell of a fight but Hearns would he wouldn't get past that jab and if he hits Manny with one of those right hands that Marquez hits him with at will, Manny goes to sleep!
The first huge right Hearns lands will drop Pacman for the count via Duran. Hearns on a whole diffeent level.
Pac started at flyweight and will end his career at welter. Hearns was a huge welter who ended his career at cruiser.
Hearns dominates/decapitates every welter-superwelter from his era to this day. Everyone. Maybe.... just maybe Oscar survives 12 rounds because of his iron chin, his decent size and the fact that Hearns doesn´t carry his power into the later rounds. Julian Jackson has a punchers chance. Tito too. Floyd survives because of his hermetic defense, but wouldn´t even try to pull counter Hearn´s jab. Too ****ing risky.
That's your problem and that's beside the point. The fact is, Hearns started at 147 and grew up even more massive, Pacquiao is maxed out at his weight, He doesn't even weigh 147 at the scales. He's at a tremendous disadvantage. Which makes this discussion already as pointless as it is.