The 4D code on the cowl tag was not used at the Doraville, GA plant for 1967 Z-24 cars. Your car may still be legit but the invoice is not concrete evidence since they can be reproduced just like window stickers. Doraville cars usually have the build sheet stuck to the inside of the driver's side rear armrest housing. Pull out the ashtray and peek through the hole to see if it's there. The build sheet or protect-o plate can document your car.