You should be able to tell an original from a reproduction just by looking at it. The condition will reveal much, look at the back interior side for older looking rust, minor surface rust is no big deal, look for an older patina on the outside. Service replacements should have a C7ZZ sticker.
www.tricityplating.com should have a 67, 68 original rechromed bumper in stock for around $300... could save you alot of time, shipping, waiting ect.