blackmerc:

The center of the mag clutch is "slipped" onto the drive arbor. That "slip" fit, however, may have originally been no more than .001", and now may be a bit corroded. If you choose to use a jaw type puller that reaches around the clutch, use it gently. Penetrant squirted into the bore of the clutch will help.
With a slim wrench you may be able to remove the four bolts from the clutch drive flange-to-pump flange connection, so that the clutch and fan are removed as one unit.