Peak Louis vs Peak Marciano.

First, I don't think a Prime battle would go the distance. Neither man had the foot work or defense to make that happen. When you factor in each man's strengths and weaknesses, it's very unlikely that a fight would go the distance.

Given the rules of boxing, along with human genetics and athleticism and the laws of physics, the closest to perfect heavyweight boxer would be between 6'1-6'4" tall and weight 200-225.

Naturally, there is a margin of error in every situation. But history comes closest to proving this theory. Anything outside those dimensions and a heavyweight begins to lose in certain areas.

Fighters over 230 begin to sacrifice stamina.
Fighters under 200 sacrifice power.
Fighters under 6'0" sacrifice leverage and punching ability. They have the power but, often not the range. They also suffer defensive deficiencies.
Fighters over 6'5" suffer from bad balance.

All the most valid debates, for the best heavyweights, involve fighters who are between these guidelines. There are very few valid exceptions.

Rocky Marciano is a beloved champion. That's why it's very difficult for many people to imagine that he might not have a chance against a "C-grade" champion, like Nicoly Valuev.

Joe Louis is right at the cusp of where the perfect heavyweights lie. That's why he would have an easier time against the bigger fighters, than would Marciano. Louis is considered small by today's standards. But don't let that fool you. If you're good enough, 6'1/205 is all you need against most of these giants. Louis is rare.

The reason Marciano could compete against Louis is that he is only about 12 lbs lighter, as Louis would likely come in around 202. Rocky also walks around just over 210 lbs. but trains down to 188 for speed and balance. He had no skill to speak of but he made the best use of the physics he had. He also got the most power that his body-type and size could allow. At 5'10, he didn't need a lot of weight, esp. with Louis not being much bigger.

FIGHT

Because Rocky and Joe don't rely much on foot work, there would be no seek-n-find. Louis would use the jab early. Though Rocky KO'd him, he was a very old 37. Louis would be younger and fresher, thus landing more accurate punches and having better stamina.

It's more likely that Louis would use his jab to cut Rocky up. It's also very likely that he would take a terrible beating to the body, along the way. Louis was suspectible to the right hand. However, he was also a master at blocking and parrying. His trouble with rights usually came from boxers, not brawlers. Still, Rocky would get in a few.

I don't believe either man would sweep a series of 3. Its likely that Louis would win by TKO in the later rounds. Its also very likely that Rocky could win the first and Louis wins the next two. Rocky was better at ducking under punches, and making himself a lower target, than people give him credit for. He was smart enough to not stand straight up to taller boxers.

It's not a stretch to imagine Rocky getting inside and landing on Joe's "above average" chin. Schmeling knocked him down and, over all, Louis suffered 10 knockdowns in his career. Its not impossible to imagine Marciano scoring a knockdown.

Given the particulars, I don't see any fight, between these two, going the distance. You have two power hitters with limited foot work. Someone's going down.

Marciano KO 13
Louis TKO 11
Louis TKO 10