I would lean towards Paul Williams at light middle, but otherwise, there is a reason why Pac v Mayweather is the most called for fight in boxing right now...

Pac being AS fast as Mayweather... Maybe not as 100% effective or intelligent speed, but he can land as fast as Mayweather... He's VERY light on his feet and is very good at ducking and taking away the target size to BE hit.. He's a southpaw which is more effective on Mayweather.... And most of all, he punches from much more angles than most fighters... And unexpectedly... Strict boxers are figured out by Floyd within 2 or 3 rounds and he makes them look awful... The fighters who can really work angles to the body and overly pressure Floyd have been the closest to beating him.. Castillo and Hatton.. Hatton was just way to hittable... Pac is about 100 times less hittable than Hatton was in the Floyd fight..
I think Pac has a few different game plans that he could execute to take rounds of Floyd... It'll be a cracking fight..

One thing that worries me though for that is Floyd looked much much bigger than Shane... Shane was supposed to be bigger and stronger, but by round 5, Floyd looked faster, stronger, bigger, taller, longer reach, more aggressive.. But all that lost most of it's effect in Round 2 vs Mosley because of the pressure... Pac will put more pressure than most fighters without being hit as much...

Pac is the biggest risk to Floyd...