I think that a clematis might not be the vine of choice for your location. My reasoning is that 1)type IIIs will require pruning unless you just want flowers only on the far ends of the older vines and in addition, in your zone, you might get some die back in younger vines from the type IIIs thus requiring pruning, 2)type IIs may freeze back all the way to the ground each year and thus require pruning, 3)type I montanas may or may not survive and bloom in your zone depending on your microclimate, and 4) any clematis growing down the face of a rock is going to have a hard time since it will be blown around by the winter winds since it won't have anything to cling to. Have you considered planting a climbing hydrangea or one of the ornamental Boston ivies at the base of the rock and allowing the sucker feet to allow the plants to climb up the rock?