Typically I've tried to take all the pictures I can of original birds over the years to document things like this. Placement, type of decal, use, California car VS no CA car etc.. I've found this is the best way to cross reference.
Then I've tried to share as much as I can with everyone here. I know many like Jim C and Vikki have done much the same. It goes with out saying that Geoff has built this site for just that reason! There should be some info and images in the FAQ section.
Much of my reference is for 67, but I have some for 68 as well..
What are you looking for? Or maybe the question is what do you have to start with?