90% of time when I’m changing from low to high I jerk the car and lose my momentum. I’ve tried feathering it but I only get lucky for a smooth shift maybe 2 in 10. Any suggestions?
If you aren’t already, try reducing the throttle just prior to engaging high to better match the engine speed to what it will be once you are in high gear. 
As you let the pedal out push your throttle lever up. The car’s momentum will keep the car from dying, and you’ll be closer to that huge jump between Ford Low and Ford High.