You won't need them Dave because you have a cable operating the handbrake across the rear axle and the new springs would move the axle back far enough. Plus you need to be careful about lowering the rear of the car. Bump travel is already limited on a Morgan and the new springs do ride a little lower already.
A Panhard rod could be a good idea however. It will make sure that the axle does not move to one side in hard cornering.