You can scan it in three slices. Create a new document that is bigger than the finished image (ie since you're doing an 8x20, create a new document 10 x 25 or so). Then go back to each individual piece, select ALL, copy, and paste each piece into the new document. Each paste will put the new piece into a new layer. Line up layer 2 with layer 1. Repeat, lining up layer 3 with layer 2. You may need to play with levels a little to get layer 2 to match layer 1 so that the seam disappears, and same with layer 3.