Hard to say how Mitchell would adapt at LW? But all things considered I would expect Ken to beat him comfortably. I am a big Mitchell appreciator (and like a Mitchell vs Winstone fight better) but even if the size discrepancy was not there I feel like Ken is just another level.
I would have to say, Ken Buchanan by a unanimous decision, Brian Mitchell is good but Ken is better in this one. Mitchell is no Roberto Duran for Buchanan.