What's the weight difference here about 15 to 20lbs ? . Regarding the distance as Johnson was a very good spoiling type of fighter and both were durable this very probably goes the distance . Unless of course Marciano suffers bad cuts which is possible but not a certainty also with johnson you've to consider if he is going to be properly motivated , also he could seem distracted and play to the crowd rocky being the busier and taking a decision is maybe a possible outcome
I think Rocky disassembles Johnson's defense over the first ten rounds and lays enough punishment to either stop him or take a close decision.
This fight wouldn't go 15 rnds it goes about 8/9,the rock would have to end it fairly sooner b/c he would lose on cuts,i like Marciano by 9th rnd k.o ..if dirty boxing is allowed I would favor JJ because that would give him the upperhand on the inside and that's where Marciano needs to be.
Marciano is always handicapped with reach in fantasy fights, but in this case Johnson reach of 74" isn't a huge disadvantage for Rocky. So who wins here? On film, Johnson is a better in fighter than out fighter. He has faster hands plus could clinch well, BUT I think Johnson had issues taking punches that landed, and was not great dealing with relentless pressure fighters. Johnsons preferred methods of attacks play into Rocky hands where he pounds the head and the body and uses his short arms to his advantage. While Johnson might counter clean / make Marciano miss a few times, he did not have the power to hurt or change Marciano's game plan. Johnson's foot speed and defense was not as good as Walcott's so Rocky's catching up or trading when Johnson attacks. If Marvin Hart could out work Johnson, and a power hitter who landed like Choynski could KO Johnson, I tend to think one of Rocky's numerous power punches would eventually find its mark. Best guess. Rocky via mid round TKO/KO in a 15 round match.
Johnson has at least 5inches reach advantage here but that is too simplistic a way to look at fights Johnson was out reached in many,many fights against : Martin 82" Kaufman Moran Ferguson McVey Kennedy Cowler 83" Russell Burns and Jeannette had the same reach as Johnson,he beat all these guys because he had more skills,and apart from the Marciano Myth Makers and your new pal Uncleturdmite. I believe most will accept that Johnson is the skilled man here. McVey was a relentless pressure fighter and bigger than Marciano = 0-3 . This is prime for prime not a Johnson who hadn't filled out his frame as he was against Choynski. Johnson's defence was not as good as Walcott's ? Marciano might win this on work rate , I don't think so ,but its a possibility but the idea that Johnson could not hurt him is laughable and just highlights your ridiculous bias here. The best antidote for the short, crouching, swarmer is the uppercut, Johnson's best punch. No short swarmer ever beat Johnson .
I agree with your overall conclusion here. On the Hart fight though, had it been fought more modernly, judges may well have given Johnson the decision. Also it shows that if you are to come at Johnson all night your face may well end up in a bloody pulp. There's evidence in this fight to support Johnson's claim to victory although there is evidence to dampen it.