I read a little nugget of info on that switch to southpaw in Sly's book. Stallone was having a private bench pressing competition with Franco Columbu (Mr Universe) during filming and his right pec exploded, requiring a metal cable insert in the shoulder and 160 stitches. This happened before they could finish the end fight scene and Stallone could only really use his left arm for the time being, so had to use that southpaw switch to enable the movie to be finished.