There has been a lot of "variety" (being generous here) with the taper that pump shafts have been made with.
Vince Falter on his Fordgarage site describes a fan shaft taper with TWO intersecting tapers - the quality assurance is that bad.
And fans have suffered from a similar issue. Many earlier aluminum fans and even perhaps some modern aluminum production have a taper "cast" in place - and no assurance that the taper is Ford design or even uniform. On some of these you can see the "flash marks" of the segment mold piece used to form the taper.
As in NOT MACHINED. Just sent out in an "as cast" condition.
Hence the advice from many to mount your fan, torque it up to the torque limit for the size/thread of the pump nose nut. Then run it for 500 miles. Then DO IT AGAIN.
What you're doing is "stretching" the taper female over the taper male by deforming the fan hub.
Check your new fan for evidence the taper has not been machined. It is reason to reject this as a replacement part.
Also check Vince's page.
https://www.fordgarage.com/pages/waterpumpshafts.htm There are several page links there worth seeing, including a page which shows the 1-1/2 inch per foot taper design drawing for the pump shaft.
Joe K