I think Spinks would beat Witherspoon but its not a fight i would bet on. It wouldnt be easy and Spinks would have to fight a very disciplined fight but he was a skilled guy. I could also envision Spoon ripping Spinks soul from his body with an overhand right and that wouldnt be out of the question.
I wish Spinks fought more guys like Witherspoon, then maybe people would realise Tyson really beat a good heavyweight in one round. My hunch is Spinks was good enough to befuddle Witherspoon too. Especially the slobbering version that wrestled with Tubbs and Bruno only to get Kayoed by Bonecrusher.
Many have said it. Witherspoon when focused was a very talented & natural HW. Spinks was greater when a LHW, but although he bilked up he did not have the natural size & power of a modern HW. Faded Holmes was not as good as peak 'Spoon. Witherspoon fighting Holmes when Spinks did beats him too. Holmes just did not have the speed & endurance-then-to take Spinks out.