Some thoughts: a 6 x 12cm = 2 1/4 x 5 inch, so to get 5x10 inch you will need at least 5 6x12cm vertical photo's stitched together.
Then consider the weight-diference between a 4x5 and a 5x7 with reduction-back and the extra weight of a 5x7 lens compared to a 4x5 lens.
Electronicly stitching would be relative easy, the analoge way may be a bit more tricky.
On the other hand, why bother to go this way if you have 5x7 and can stitch 2 5x7 horizontaly together to a 5x10 ?????
You will need a lens that covers 5x10 anyway and a camera with enough rear shift.