I doubt that the PM alternator would be a problem.
 

        If the separation were a few inches, I would agree. However, the separation might be as little as a few millimeters. The stray field from the alternator decreases by something like the fourth power of the separation distance. As you get close, the field gets strong very quickly. The sensor will not be perfectly shielded and will undoubtedly have some presented area.

        I would put a scope on the sensor with the alternator in place and functioning, just to make certain that there was no significant interference.