I think Gene of the Heeney career finale pulls off the decision. With his jab, he might even be able to outwork Rocky in some of the rounds. His reach matched that of Louis, and surpassed the reaches of all Marciano's other ATG opponents. Both were extremely smart operators in the ring, but I think Tunney's mobility, toughness, smarts and effectiveness at long range are the difference here. Rocky would simply run out of time. The championship distance wouldn't be enough for the Blockbuster to overtake the Marine on the cards.
A prime Tunney whips him. Better proven against better fighters. Teak tough. Skills and physicality. Stylewise Marciano never figures him out, never gets the chance.
Marciano pounds Tunney into dust. Rocky has far better and deeper HEAVYWEIGHT experience and didn't feast on old middleweights and has-been light heavies to pad his record and boost his rep. Tunney would take the early rounds while while the stronger, more durable Rocky bides his time, slowing Gene down with hard thumps to the ribs and head. Despite his inflated reputation, Tunney wasn't some defensive master. Marciano by KO within 12.
If fairly washed up Dempsey could get Tunney to the "long count", a peak Rocky, who would not tire, wins.
If it was a 10 round fight I can only wonder what pace Marciano would fight, either way he is too relentless and strong for Gene and I think Rock catches him and breaks him down, if he dont catch him over 10 I think he puts Gene on his ass over 15 Genes donions