They are the factory '74-'76 Dart/Duster fender braces.
I had to trim the passenger side end a little where it mounts to the firewall since the earlier firewalls don't have the wide tabs on the seam weld where they are intended to mount to. But there is plenty of width to center a 5/16" bolt in a 3/8" hole once you trim the bars a little.
It's a little tough to drill because of the multi-layer metal thickness and you need to adjust the bar position as needed to miss the spot welds. I clamped a piece of scrap wood to the underside of the lip so when I drilled through I wouldn't scuff up the fire wall below when it broke through.