Are dart sports mostly gone

DionR said
Stiffen the front of the car. User @goldduster318 added them to his early car and notice a good bit of difference as I recall.

I plan to add them to my '73 G3 swapped car at some point.

Yes, I added them, to me the car feels better with them than without them, though small, you can grab them and rock the whole car by them.

Kern Dog said
Ford cars had this in the 60s.
There is a tendency for the front section of the car to shift diagonally under some driving conditions. The bracing is meant to reduce that.
1973 was a year of major change to the Mopars. Brakes got better, steering components improved, great efforts were made to reduce vibrations in the B body line but those were a big step backwards regarding handling due to deflection.

That's right, they keep the box structure of the front of the car from parallelagramming. It could have some minor effect to keep the upper part of the inner fender from flexing in the bending direction across the spot welds that sometimes crack.

If you can't find them you could replicate them using barstock and some round tubing, just crush the ends flat with a press and then weld it to the bar stock for the tabs. Seems to be essentially what the early construction was. I had to make the firewall side tabs to mount on mine as there are no flange provisions on an earlier car.

Doing a tie across the middle might be a good thing also, but really I don't know that I could make one that looked okay and actually fit, lots under the hood with A/C and EFI.