This fight perplexes me when I think about it. The volume punching and reach advantage make Williams a potential nightmare for PBF, but Williams nightmare is someone who can KO him (Forrest-Mayorga style) so I have never been able to venture a guess on this one. My only thought is that PBF would probably figure Williams out in the mid rounds and pull out a victory. The only reason I think this is because he does this in EVERY fight and so it is the only thing I can bank on.
Williams has a decent chance against Mayweather. Floyd would have to attack and be very aggressive which is not his style at all. He has the heart, skill and athleticism to pull it off though.