The basic layout is you have a hex shaped oil pump drive shaft about 8.5 inches long, it should have a retaining clip on one end. The end with the clip goes toward the distributor. The clip doesn't hold the shaft into the distributor or oil pump, it's there to prevent the shaft from lifting out of the oil pump and engine block and falling back into the oil pan when the distributor is removed. The shaft sits in the hex hole of the oil pump and then the pump is installed from the bottom of the engine. The distributor drops in from the top with the hex hole (in the bottom of the distributor) engaging the top of the oil pump drive shaft. The drive shaft "floats", nothing is "pressed" together.
If your oil pump shaft is out of the motor, but the oil pump is installed, then you must be missing the retaining clip. It will be very tricky trying to drop the hex shaft in from the top of the motor. If you miss the oil pump, (clunk) it will fall into the oil pan.
If I have guessed your situation, you could wrap a little tape around the hex shaft to make a friction fit into the bottom of the distributor, then lower whole assembly into the motor carefully and hope the other end of the hex shaft finds the oil pump instead of falling into the oil pan. Of course the "proper" way to do the job would be to remove the oil pan, remove the oil pump and place the shaft in the pump and reinstall everything. And use a shaft with a retainer so it doesn't come out with the distributor again.
You can see the retainer on the shaft in the attached photo.
Did I guess right?
