Quit making excuses, trust the man and get to work!
Just from analyzing photos of the car I completely agree with the rear spring rate increase.
That bar looks like it could control the USS Missouri in rough seas.
Can you make a hole saw cut forward and away from the stress zone on the frame horn that would allow you to feed a thick washer and nut to the mount bolt location with piece if bendable wire? Hot melt glue the washer to the wire to position it, same with the nut.
