I try to keep away from some of the what if this guy fought that guy scenarios as it can drive you as mad as the is there a God question!
But this thread is a bit more of an open question than a direct comparison between two fighters.

I was watching Marciano on the train on the way home last night. I think he was a lot more clever than he often gets credit for , what he lacked in polish he doubled in balls, so he made up for fighting the more stylish fighters by not trying to match them but simply outwork them and by also doing the unexpected like mentioned before he could simply beat your arms to death to get you !

If you look at the usual things like training techniques and nutrition and even the ability to study oponents like you can in the modern era, then I think he holds his own with any of the Cruisers.