1/8 good to use.

The 70-73 Duster rear brackets are about .3" thick, which is what I measured on my car. Probably 9/32" plus paint. Not sure about the Duster front brackets, but for a 70-72 dart the front brackets are stamped sheet metal. It's 9 or 10 gauge, I measured mine at ~.14" thick. So probably 10 gauge plus paint. That's a single half of the bracket, they'd be double that where they overlap at the frame. Funny enough doubling them up would make about the same thickness as in the back, might be something to that.

As far as using the 1/8", it will hold the bumper on, but you won't be able to use a bumper jack (not that you'd probably want to anyway) and they'd be pretty much cosmetic. They wouldn't do anything other than flatten out in an accident. The frame rails are fairly thin, but there are 2-3 layers of overlapped metal where the brackets attach. Minimum of 2, because at the very least you have the rear cross support and the tabs on the frame rail. And then there's the fact that that rails are basically tubular, and since the brackets span the whole rail and then some the surface area covered becomes a big part of the strength, so it's not just about the thickness of the metal.