Is this the correct procedure?

Brass plate on door
Plastic plate on qtr. post
Shim under plastic plate for 1/32 interference

Purchased shim kit from Cincy Auto Trim and the shim hole spacing fits the brass and not the plastic. The shim is shaped like the plastic plate. Fisher body shows the above.

Pics worth 1000 words...I don't know the answer, but this guy might. Or might not. He says the shim goes under the brass...different than you wrote:
http://www.camaros.net/forums/showthread.php?t=108061

I can tell you on my 67, the shims are under the brass plate on the door. So far the car has turned out to be pretty unmolested so I believe this is how they came from the factory.

I agree with hokie.
At least on my '69 the shims are behind the brass plate on the door.