The Hawk would only have a puncher's chance....a GOOD puncher's chance mind you...but a puncher's chance nevertheless. Manny would crucify him outside of getting hit with a fight ending bomb.
I'd favor any of the good/great 154 pounders to beat Pacquiao. Margarito wasn't throwing enough punches and had opportunities to do so. He was hurting Pacquiao badly when he did land.
Manny has shown an excellent chin fighting bigger guys but he could not afford to get hit clean by Jackson. It's not as Julian would knock Pacquiao out with the first semi-power shot he'd land, but Pac would have to concentrate much more on defense than in any other fight.