One solution is to make one side of the spring mounted solidly to the batwing........don't use a shackle on that side. I think Speedway sells a version for spring over axle installations, they call them something like a "dead perch." It would be easy to make up a similar mount out of flat stock so it would bolt or weld to the batwing, drop down a little to simulate the drop of the shackle angle, and then have a long bolt going through the mount and the spring.
The spring would not swing on a shackle on that side, and it sort of turns the spring into a panhard or locator to keep the axle from moving right to left when you turn the wheels. Not quite as effective as a true panhard, but it is a means to an end, and certainly better than no control in a cross steer car.
Don