I think you mean that the dove cloud in TG extends towards the horizon, lies 'flat' above the ground, and you want it perpendicular to the ground. In sky you should be able to watch and work on your painted cloud from another cameraview, and maybe move it the way you want. I'm not exactly sure how that works, but it should be possible.
But I think it's easier to work directly in Terragen; make a Photoshop mask, project it by a new camera (or the render camera) on a thick and high cloud in front of the render camera, and use a narrow simple shape perpendicular to the camera to clip the cloud to say 1000-1200m from the camera. If you use a second camera, you can move the projection around a bit as needed. Oh yeah, and in tweaks, uncheck taper top and base for a higher cloud.