The problem is not logical at all.
Is there possibly an axial play and as you mount the old pump pushes the shaft outward?
It seems there is about an 1/8" clearance missing, too far out, I could be wrong with the distance.
Are the mounting flanges identical in thickness?
Was the new pump not turning or the old one?
Is the pump housing (forgive me for asking) in the correct position?
Is the pump perpenticular to the motor block?
Since the problem now also exists with the old pump, the lenght may be correct.
Now, this is far fetched, is there a shim inside the pulley?
I think there is some weird seating problem, if all dimensions are the same.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert