Not all Pontiacs had the shim, I've put a good many of them together without it and never had any ill effects not using it. Pontiac is the only manufacturer that I have seen use a shim between the flexplate and crank flange, and I have no reasonable explanation of why they used it in some instances, and not in others.
It may have been just a way to take up for manufacturing lateral tolerances, I can't say for certain why it is used in some applications, but not all.
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Brad Yost
1973 T/A (SOLD)
2005 GTO
1984 Grand Prix
100% Pontiacs in my driveway!!! What's in your driveway?
If you don't take some of the RACETRACK home with you, Ya got cheated 
  
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