I never got a straight answer from anyone with a Procharger and my year(s) mustang. After installing the BGG spacers on the s/c bracket and grinding the alternator face; BOTH for alternator pulley and s/c pulley clearance it is definitely required to mill the alternator boss down. As you can see in my pics the alternator pulley outer (front) ribs are out by one maybe two ribs. Here are some pics. Hopefully this answers fellow 96-98 GT questions when doing this modification and saves you some belts and your minds. Off to get my pulley milled and I will repost and after pic.
you can see it barely sticks out a little bit
you can see it barely sticks out a little bit