It is vital that the optical axis of the Vidicon is at right-angles to the picture plane and that, whether it is the Vidicon or the painting that moves during image capture, it remains perpendicular. Obviously, with a seriously warped panel, this may not be possible. The lighting should be carefully adjusted so that the area of interest is lit as evenly as possible. If you can, arrange for the lights to remain stationary with respect to the camera.
An easy way to test camera alignment is to image a piece of graph paper, move the camera (either left-right or up-down) by 90% of the field of view, and see how features in the overlap area move. First rotate the centre around the optical axis to get the centre line of the images lined up. Next check the corners and adjust camera pitch and yaw.