There are a couple "gotchas" on the 71-73 parts.
The taillight panels are pre-punched for the 73-only center bumper filler panel - on a 71-72, those holes need to be welded shut. It's the horizontal row above the bumper mount holes. You also want to verify the brackets for the taillights are welded on correctly. I've seen a couple instances where they were crooked and the car was already painted.
Some quarters also come pre-punched for the 73-only quarter extension bumper filler piece. Again, those holes need to be welded shut on a 71-72.
Another bit of advice is to tack or self tap all of the panels in place and dry fit everything before welding. You'll want to have the valance, quarter extensions and trunk lid all installed to make sure everything lines up correctly.
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