Southpaws Guerero and Maidana (second fight) landed 113 punches and 128 punches on Floyd, respectively. Those southpaws are not as fast, as busy and as unorthodox as Pacquiao. Pacquaio against Bradley (second fight), Mosley and Marquez landed an average of about 200 punches. I believe that Pacquiao will land anywhere between 150-200 punches against Mayweather, most of those hitting flush on the face, and I believe that would be enough to keep Mayweather on the defensive all night.
Not only will Pac land on Mayweather, he's going to bust him up. Mayweather will be in deep waters he's never been in before and we will see his true colors. I don't think Mayweather likes adversity and becomes child-like when things aren't going his way (Maidana fights).
I think Mayweather will be forced to open-up late and trade with Pacquiao after his father stutters the words, "uurrr uurrrrrrr urrrrrrr fallen bee bee beeeehind, knoo noooook dat muu mu mufaca out!" That's when things get interesting and possibly Pac stopping him in the sickest way.
^_^