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Do you have the distributor out already?.
If so the the shaft should have stayed in the oil pump.
If the shaft has come out with the dist. Than you are going to have to use a piece of thin paper or heavy grease to pressed between the distributor and shaft, to keep the shaft from falling off before it ingages the oil pump.
There should have been a retainer on the shaft when the oil pump was put in the block, but doing the course of things, someone might have left it off.