Depends on the question... do we rate him amongst his peers and guys similar in weight? cause 175lbs-195lbs he's a killer, above that it'd require some good matchmaking and in the 230lbs+ territory he has very little hope today against the dreadnought's.
Top 10 is a high bar. I have him at World Class based on the options provided. I would suggest another option above World Class where I would put him - perhaps "Hall of Fame", or "Top 50".
Pretty much this. All my slagging on Marciano is largely aimed at his worshippers. He's a load at the old cruiserweight limit. Head to head there, I would rate him quite highly.
Stick him in the modern day Cruiserweight era which is what he should be at because its only fair he'd rate highly in that regard.