[LongBox]

The shims that I am familiar with were sort of horseshoe shaped. All the 60's and early 70's GM cars and trucks used them. They usually had a turned up end, so you could push them in or pull them out to get just what you needed. I had a large pile of them when I used to work on old A bodies, but figured those days were over, and threw them out a couple of garage clean outs ago.

If you can get a pile of those shims, you should be able to get good panel fits by trial and error, adding and subtracting shims.