[Mart]

If you add a panhard rod, the shackles are relieved of any sideways location duty, and can be as long as you like, within practical limits.

I have long shackles and a panhard rod on the back of my 40 coupe. The shackles are about 6" long.

Not sure if a sway bar can be trusted to the same degree as a panhard rod, but The original fit 40 style bar does offer some degree of sideways resistance.

Mart.