if you watched the marciano-walcott fight you would know that marciano was being out boxed and out hustled by the bigger walcott. although walcott was dominant in the fight, he still took some HUUGE RIGHT HANDS AND LEFT HOOKS DURING THE FIGHT WHICH SURPRISED ME THAT HE WAS STILL STANDING.
eventually them shots caught up with him.

now as for roy, he couldnt even take a shot from tarver hu hits no where close to marciano. roy may be good enough to out box marciano but no way would he take marciano's shot, and in a fight you have to give and take shots. marciano would possibly give roy brain damage, rou doesnt have the defensive moves or the chin toney has to fight bigger guys!