To me a case can be made. Started out as a middle, beat Archie Moore twice, making him the greatest LHW ever, and won the heavyweight championship. Beat two heavyweight champions, three LHW, and a host of good contenders.
I'd definitely have him down as ONE of the greatest fighters of all time. A very good heavyweight and a great light-heavy. Pity he never held the title in that 175lb division. He'd have held it for a quite a while.