Try swapping the nose cone of the old starter onto the new one- - - -just be sure the bushing that supports the front of the armature shaft is still good. That small bushing has a habit of wearing out and allowing the armature to contact the field coil pole shoes, resulting in a "dragging" starter that doesn't come up to full speed. It's not terribly difficult to swap nose cones, but the solenoid will have to be removed during the procedure. Disconnect the terminal that goes into the starter housing, remove the two screws holding the solenoid to the nose cone, and twist the solenoid 1/4 turn to disengage the front tab. There's a pretty stout spring in there, so don't let it catch you by surprise!
Jerry
