I'm considering an A 500 4 speed overdrive for a 72 Duster

The floor shifter is a very important thing for this car.
I had a Gear Vendors unit in my '70 Charger. It was an early model with no mounting boss for the floor shifter setup. The prior owner of the GV made a bracket that was secured by a bolt in the GV case. I suppose that with some thinking, I could fab up something like that.
If not, a cable arrangement could be used.