Starter woes

Mopar starters don't need shimming. Neither do Ford's
That's a GM thing
Look at exactly how a GM starter mounts vs how a Ford or Mopar starter mounts. "Shimming"a Mopar starter only keeps it from engaging as deep on the ring gear. Not changing the clearance between gear teeth one bit. To shim a GM you're pushing the starter away from the side of the ring gear. To shim a Mopar you're pushing the starter away from the face of the ring gear.