I don't give Young as many rounds in this thing as everyone else does. Jimmy was a cagey fighter, but Marciano, in his hungry way, was a very cagey guy too, like a lion at feeding time; the Rock's instincts are keen, he gets to this defensive fighter early on.
I am going with Rocky by KO/TKO in this one. I also think Rocky would be ahead on the cards at the time of the stoppage.
Marciano by brutal KO --- early. Check Youngs record. He lost to several guys that were much smaller and many of them were journey men. This fight would not be competative.
Marciano is the wrong style for Young, as Joe Frazier said " dont pull a rope a dope with Marciano" he would beat your arms and shoulders" and in this fight he would do just that and his workrate would get to Young ...Marciano KO Young