A 275/60/15 (28") will fit under a 69 Dart with some work (springs and offset).
With 26" Hoosier drag radial my 69 Darts best was a 1.889 60' 4 spd 3.91:1. And the car is crossing the line at 54-5500. I believe the set up is struggling to get the weight transfer, so I tried a set of gears...
Compare 4.30:1 with the same set up, and the car was out of breath at 1000' of track.
Save your $ and work on getting the weight to transfer, and your fuel system to keep up with the load a launch.