I honestly don't think Diaz can even handle Manny Pacquiao. That's not to say Diaz is a bad boxer. It's just that the best boxer in MMA would be average in pro boxing.
Thats the fun of it....ANY boxer, past or present prime vs. ANY MMA fighter. Catch is: Pick a MMA fighter who would mesh well with your boxer. How about, Liddel vs Marciano. Both guys would trade blows.