73 Duster build named Fred

I get what you are asking about the stall speed. I would like to mention, my car/engine is similar to yours and I 60' 1.42. My converter flashes around 5k as well. It has not changed with 2 different converter's, a turbo action that I got from and one I had redone by PTC that was a turbo action that I had broken. I am foot braking, no 2 step or t-brake. 3k launch.

Edit to add: You have more motor than what I have and I feel that you should be 1.40 or better. Also my 60' didn't change when switching from super stock springs to the Calvert set-up.