no you dont have to cut open the transom to do it.... what needs to happen is on the starboard side of the boat at the rear, there is an access hole. take the foam out of this area and climb in with the wrench to hold the hardware while someone on the outside does the turning....
im 6ft 210lbs and ive been in there. go slow and methodical and dont panic. you can do it.
but I do have to say that for me, its much better when my 5'2" wife crawls in there instead......