If you are talking about replacing the oil seal where the shaft comes out of the gearbox, you don't have to remove the engine. I recently replaced mine (2GM) and it took about 1 hour. What you do is tilt the rear of the engine upwards. You'll have to remove the engine bolts and uncouple the shaft. There will be some play in the shaft that will allow you to push it an inch or two down and out of the way. If there's not enough room, you may have to lift the engine up just slighly off the engine mount bolts. We used the main halyard to lift the engine and other sheets and blocks to adjust the engine forwards and backwards as needed once it was off the engine mounts. There's a single large nut that attaches the coupling flange to the shaft, once that is off you simply pull out the old oil seal and replace it. To keep the shaft from turning while loosening the nut, I put screwdrivers thru the flange holes thru to the back of the gearbox where there are holes that will line up - although there's likely a better way. Use a torque wrench to put the nut back on. I forgot what the torque setting is, but its in the repair manual. Let me know if you'd like me to look it up for you.
As far as your stuffing box is concerned, you should replace the stuffing every few years and check the nut for tightness to keep from leaking. The teflon coated stuffing seems to work pretty well. As for replacing the box, if you can stop the leak by replacing the stuffing, why bother replacing the box? Good luck!
