Ali would not absorb a great deal of punishment then. It would not be very different than against Liston. Ali never passed a physical after Manilla, & was clearly showing early signs of Parkinson's disease by the time he fought Earnie in the Fall of '77. Up to Manilla Ali would clearly win an easy decision or very likely KO him-Shavers conserved his energy & was still out on his feet t the end of their actual fight. From '64-'67 against prime Shavers maybe Shavers does better than Cleveland Williams, but I do not think he has the stamina to last as long as say Terrell-who Ali toyed with like Patterson.
Wouldn't be a close fight, Ali would dance around Shavers and pick him apart with the jab and combinations. He'd likely put him out in the middle rounds after wearing him down with many combinations.