well the more original info you have, the better, is an easy rule of thumb. "important" can mean so many things, and be different for different people.I would break it down like this: Original build sheet- probably one of the most sought after pieces of documentation along with..
protecto plate- another valuable tool in checking a car for originality
provenance from dealer to current owner- yes nice to have or know, but not as important as the previous 2. (If you don't have the previous 2 than obviously more important)
So IMO the protecto plate would be number 1 followed closely by the build sheet.
Another nice one you didn't mention but some really fortunate guys have is the window sticker.
How about some pics of the 67 you just picked up?
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