Whilst Duran started to go off the boil as a welterweight, that was far more due to loss of desire to train like a maniac, than an inability to handle the weight. Pac's chances against Duran worsen at a heavier weight. Anyway, Duran by KO at any weight from 130 and above.
Just watch round 2 and 3 against Leonard (1st fight). Imagine Pacquiao doing that against leonard. you can't. Too tough, too heavy handed, too intense. I would say Pacquiao is faster but look how Duran handled a faster fighter in leonard. Cut the ring off superbly.