Here's an image of a cross section. You can see the groove inside up where the bearing would sit - at the top, adjacent to the belt grooves. There don't seem to be too many spare millimeters at that point! The second image is of a turned down 68.5mm pulley. I am unsure if they are related images. Turning down 3.1mm each side from 74.2mm stock will yield 68mm. I'd be reticent to go farther than that. If the cost of turning down is not too high then I'll sacrifice one pulley to the cause. (If I can find the owner of this image of course, I will ask them the dimensions of the cross section).

C32superchargerpulley_crosssection.jpg

turneddownoemscpulley685mm-1.jpg