Mosley wasn't at his best as a welterweight he was clearly better as a LW. However a lightweight of his era is the welterweight of earlier eras, so Mosley would have the punch and the chin he had as a LW. Nobody ever wants to go there however since it might mean that the welters of today might be difficult to handle for the welters of the past because of their size
In a case like this, I just use the lightweight Mosley You could put Welterweight Mosley in there against Junior Middleweight Benitez, if you'd like... The winner would still be the same, to me
Prime Mosley was pretty useful against sluggers. First competent boxer to actually box him was Vernon Forrest. Benitez outboxes him clean.
Doesn't matter if Shane comes into the ring heavier than Benitez. The gap in ability and skill is too wide for it matter. Benitez totally bamboozles Shane.