Man that’s a tough one. I could see two possible solutions to your problem, but they are basically just guesses as I haven’t experienced anything like that or read about someone with that extreme of a difference from base to head.
First, you could just focus on girth work, specifically targeting the upper shaft to even everything out before doing anything more than manual stretches.
Second, you might try attaching a hanger high (towards the base) where you are thickest to focus the stretching on that area of the shaft.