what is the ride height on a 71 dart and how do i determine this height

Got a factory service manual? You need to measure in two places - from the bottom edge of the lower ball joint, by the grease fitting, to the ground, and the from the part in between the sides of the lower control arm that the adjuster bolt pushes against (the 'blade') to the ground. The blade should be 1 1/2" higher than the bottom of the lower ball joint. Adjust the height by tightening or loosening the torsion rod adjuster bolt, then bounce the front end a few times and re-measure, and then make the heights the same on both sides. Caution - adjusting the ride height changes the front end alignment.
So there is no set ride height, per se, because it can change if the springs sag, etc.

I always measured to the fender lip right above the
spindle centerline... I dont recall the measurement
but when I was part of the design team for the transverse
torsion bar suspension we used the Darts as the
base for the suspension testing(mule cars)... I would
have the front end out 2 or 3 times a day making changes
so I used the fender for a quick and easy measurement