Williams is not just a southpaw, he's a 6'2" southpaw with 79" reach
Mitchell is 5'7" with 72" reach, recently stepping up from 140 at 35 years old.
Williams is younger, fresher, bigger, stronger, longer, taller...he's also very well conditioned and has a solid chin, not that he'll need it in this fight. Mitchell didn't have any power at 140 and Williams is very big at 147, sometimes fights at 154. Mitchell was stopped by Tzsyu and Mayweather, I expect him to be stopped here also. He'll have to use alot of energy and eat some serious leather to get inside the 7" height and reach advantage, Williams has good hand speed and plenty of power to hurt Mitchell coming in. I don't think the old and fading warrior has any chance of winning this fight.
Just my opinion though
gl bl