to start with, I would leave the short arm where it's at. I would remove the long arm, shorten it, and attach it to the same place the short arm is.
It still won't be exactly right. because the two arms are different lengths, the arc will be slightly different. But I think the difference would be so subtle you wouldn't notice.
If you want it to be exactly right, make a new pedestal for the bell crank in the exact middle of the manifold base. Then the two arms would be the same length, and move through the same arc.
