I'm a '67 too. I got mine from a little old lady who was the original owner--no kidding. It was a sad story--she had a stroke and I bought it from the estate for 2500--it was her daily driver up to about a year before I bought it. I even got the original build sheet, bill of sale, etc.
Don't know how to tell if a car's orig or not, but getting one like I did helps one be sure it's pretty original. Mine needs everything though--although nearly rust free, windscreen channels were a mess, as was suspension/brakes/steering/interior. Plus had an earl shribe paint job at some point which is a disaster.
As one of the posts stated, '67 was a good year 'cause 2 saftey things became standard; dual res. master cylinder and shoulder harness mounts.
I had a '64 (my all time fav chevelle--too hard to find now though)--but it had the old single res master cylinder (yep, I lost the brakes once and it sucked)--and no mounts for shoulder harness. Some (all?) '67s have the shoulder harness mounts in place, even if the car didn't come w/ the harness.
Enjoy