Oil pump shafts should have a lock washer near the top of them. In a normal situation, you install the shaft from the bottom and the washer is larger than the hole in the block where the distributor goes through. This keeps the shaft from coming out when you pull the distributor out.
You are correct that you will have to be very ginger with it and it will probably be a little difficult to install it from the top once the engine is upright. Not impossible, but much harder.