You should be able to replace the stuffing box seal (rope) from inside the boat with the shaft still in. Check out u-tube videos.
Some even claim you can replace the stuffing box seal with the boat in the water. Clean the treads and use some penetrating oil before attempting to back off the compression nut
If you want that shaft coupling off, first - if you can, separate it from the engine. For me, the coupler bolts were as much of a heart ache as getting the coupler off the shaft. Back out the two bolts that hold the coupler to the shaft. Again another fun task as the coupler is cast and the bolts are steel
Next go get some high end penetrating oil. (some buddies that own a machine shop gave me some. Don't know the brand but it was orange in color) Soak the coupler/shaft over night. Get something to insulate the shaft/coupler from the hull and then heat the f**k out of it with a propane torch. Get a 5 pound hammer and have at it - good luck
