Another way you could try to do this. Print each print individually to the required size & join them up on a light box, tape or mount them together. Then trace the outlines of the entire image & transfer the tracing on to a stiff board like foamcore. Use this board with the traced image on it as your easel. Print each neg on to your paper by first lining it up exactly with the traced image on your "easel" & then expose at your pre determined exposure. I'm not sure how well this would work, but I have blended two negs together using this technique, with limited success. You can also soften the edges where the images overlap to get them to blend together.