Leon Spinks at the top of his game would be possibly too busy and strong for Micheal... who would have too try and out box Leon.
At his best Leon was a very busy and agrressive fighter with a good chin and decent punch and I can see him winning a decision in a good slugger vs boxer contest.
At heavyweight, it's hard to say, because their careers as world-class heavyweights were both so incredibly short and confusing. I supose I'd have to favor Michael.
Mike would win this fight. the only common opoonent they have his holmes but holmes was past his prime when he lost to michael. I still think michael was the better boxer and would have outboxed leon