Same problem on my '71 big block 727. The carbon fiber around the mounting hole was hitting the trans case lifting the shield slightly to where the holes were out of alignment. After fighting the shield for a few hours, I just took it off for now.
Drilling the inserts as mentioned sounds like it will work for me. I was thinking of grinding more clearance in the trans case too?
Anyhow, I'll mess with installing the shield when I get the car lift