67 Dart tail-light housing question...

Why can't the 68-69 repro metal housings be used on the 67? I'm referring to the sheet metal. I thought the 67-69 lights were the same except for some lens styling so I'm confused on why they list that repro body metal piece as 68-69? My metal in that area isn't in the greatest shape and I would rather have the body guy use a repro price than spending time and money on fabricating that.
Any answers are appreciated!