I'm inclined to saying Floyd Patterson. Neither was a very big man in that weight class but Patterson was more accustomed to fighting true heavys and faced his fair share of quality ones. Not sure that he ever really got a good gauge as to good Michael was at heavy.
This would be a good fight, awkward boxing skills vs. great speed. Sure Patterson was more proven at HW, though HWs he fought were somewhat smaller on average compared when Spinks campaigned there. And in actual height & weight-using the Tyson fight-Spinks was almost identical to a prime Ali & lean. Tyson was Spinks' Liston. Both had trouble vs. the scariest slugger. Hard to haver a strong favorite. Possibly Floyd, 60-40%.