It's not so much a conventional/southpaw thing. That just effects which way you pivot and which punch you throw. For example, if Floyd's on the ropes covered up and none of Manny's punches are getting through, one tactic would be for him to swing his back foot out to the left so he's perpendicular to Floyd rather than straight on. After kicking out to that angle, there should be openings to the body from lead hooks or to the back/side of the head from overhand lefts that weren't necessarily there when they were fronting each other.
The difficulty was more Floyd's defensive posture. Some fighters against the ropes maintain their stances because they want to be in a position to punch with power. Of course, in their regular fighting stances, they remain open to their opponent's punches, but now are entirely reliant on their gloves and ability to duck and slip, because they don't have any room to maneuver with their backs against the ropes. What Floyd does his front his opponent or put his back entirely against the ropes. Instead of being in a 3 quarter turn like a normal fighting stance, he faces his entire torso and head forward and covers them both with his gloves and tucked forearms. His back, liver and kidneys are protected by the ropes and unreachable. While this cuts his ability to punch effectively, it essentially ensures that no punches will get through. Even if Manny popped out to an angle, Floyd is still almost entirely covered up so there would be no new openings to exploit.