"Keep in mind I am no expert". I learned to do this when my 356 had a leak in the area where the shaft enters the transmission (car now sold). But, the principal is the same, get the old seal out without scoring the shaft or (be extra careful) scoring the aluminum housing.
I have an array of picks and bent flat bladed screw drivers I have heated myself with the torch. The screwdrivers have angles from 45 degrees to 90 degrees. Needle nose pliers as well.
Start working the rubber seal, one side then the other or work in 1/3 rds around the circle.
DO NOT GOUGE THE INSIDE OF THE ALUMINUM HOUSING !!!
I could take five minutes or five hours, take your time. The 356 took a total of four hours on my back as it had been in the trans for 40 years.
Once the seal is out, inspect the steel shaft for sharp edges, gouges, cuts etc. This is best done with the shaft brought out as far as it would come towards the rear. I do this with one of my many dental tool by sticking the handle in the hole, giving a turn right of left and pulling to the rear. I had a slight sharp scratch. Extra fine emory cloth, (one inch wide) took care of that.
I grease up the new seal and slide it on/in............"geez Joe, sorry, I know it's a family site".
If someone has a better solution, I'm all eyes.
mike