What about if you unbolted the 'C' shaped plate, looks to be about 1/2"± thick and bolted a vertically adjustable clamp in its place. You'd have to custom make the clamp as the bottom plate would have to be 8" or so long.
Another idea would be to have a piece of 2" or so round bar the full length of the blade with a square machined on each end. These squares would slide into a slot. Instead of being on the centerline of the bar the squares would be offcenter, an eccentric for lack of a better description. When you turned the bar the distance from the base would change depending on the rotation of the bar. It's the same principle that a Scothman ironworker uses for the shear...
Mike
