I don't know about the 4 strokes but I just replaced a prop shaft in a 1987 25hp Merc & it was a walk in the park.Didn't even have to remove the lower unit.Carrier retainer is left hand.
wow! i cant believe it was that easy! the hardest part was getting the retainer spring that keeps the pin in the shaft. no where can i find a new shaft and not get shafted