We've achieved low 1.5x 60 foot at 123mph with Rancho adjustables. They need to be set tight. Try about 15psi. I would think at 120mph you should be able to go low,1.60 high 1.50s. Make sure the shackle doesn't limit travel. You may need to be moved forward or made longer. Better yet switch to sliders. Make sure the shock is long enough so it doesn't limit travel. If it does add an extention to the mount. Shock angle at this stage of the game isn't really a factor. Go to you tube; "64 Dodge shock tuning"
https://www.youtube.com/watch?v=sjZDXBXRC9QDoug
