Tunney was a master craftsman and incredibly intelligent boxer. His defensive ability and counter punching skills were very special to see. Rocky Marciano of course the complete opposite of Tunney in every way. Rocky was the definition of a blood & guts warrior. He could take your shots and deliver his own venom which shook you to the core. He was definitely dangerous when it came to brawling.
The only way I think Tunney can prevail against Rocky is to Stick & Move and not let Marciano lure him into a punching match. Tunney would have to use his superior ring IQ to be a step ahead and befuddle him. Hopefully Tunney's stamina will not betray him. If he does all this successfully he'll win a decision.
Rocky would have to cut off the ring and bull him to the ropes. The less ring room for Tunney to run around in the better for Marciano. That means Tunney would feel his power much more.
Whoever is more successful with their game plan will win the fight.