Doug,
The top of the oil pump shaft should have a taper, as does the bottom (inside) of the dist shaft, thus allowing the two to "find" each other.
Measure the hex in the bottom of both of your distributors. Maybe the new one is smaller, or has a burr on it. Try inserting an allen wernch as a test fit. Also measure the OD of the old and new dist shafts at the oil pump drive end.
John
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