There is obviously no way the get the shaft and retainer installed through the top via the distributor hole. If you want the retainer (highly advised), you pull the pan and the oil pump.
Installing the shaft from the top is best done with a 1/4" deep socket and a piece of tubing that fits over the socket extension bar. You will use the tube to push the socket loose from the extension when you have the shaft in place. You remove the retainer and lower the pump shaft into the pump via a ¼” deep socket with some Scotch tape on it to hold the pump shaft in the socket. Once the pump shaft is in place, use a screwdriver or tube to detach the socket from the extension. Use a long thin screwdriver or steel rod through the center of the ¼” socket to hold the pump shaft in place while you use some mechanical fingers to retrieve the socket. This will prevent the pump shaft from lifting out of the pump socket and falling into the bottom of the oil pan.
If you successfully use the above method, you may be faced with the necessity of removing the pan the next time you remove the distributor. There will not be anything to prevent the pump shaft from lifting clear of the oil pump socket and falling into the bottom of the oil pan or wedging itself in the crank and rods.
