He wouldn't KO Ray Mercer. In fact, the Ray Mercer that turned up against Lennox would give him all he could handle... and possibly more...
Tarver is a good shout. It is amazing how similar they are in some ways. Can't completely put a finger on it, but there it is.
Marciano was not some bum and is a LOT of levels over most of these guys. A great little man will beat a decent big man.