Curved horizon ..help please
You could download Panorama Tools and PTAssembler the front-end (GUI) - both free. It may take a little bit of learning but very good for what you are trying to do. Also there is an requirement to set a reference point in the software - how high or low the reference point is placed makes a huge difference in whether the stitched output comes out with a flat horizon or a curved one (up or down).
I used it to stitch 13 hand-held pictures which came out without any distortion that one can see.