Past experience has proven to me that you really do need the mid stay for the clamshells.
The wings are subject to pretty high pressures and vibration with the result that I have seen extremely large holes appearing where the rearmost rivnuts have pulled out.
I have also seen cars with wide track and clamshells with the wings spaced out from the main tub. I suppose that, if the spacers are not a continuous wing length unit, it would slightly help relieve the pressure?