In real life there is no physical reason why the arch could not have a wider span on the outside of the viaduct; it would have been accommodated by very slight adjustments in the brick courses. However, if you intend lining the underside of the arches with printed brick sheets you would have to accept some mismatch, same as if you went for stone courses. In the extreme you could cut several narrow strips of stone and probably get a very good resemblance, but given the number of arches you would need the patience of a Saint. The far easier option [to build] in my opinion is to have constant span arches as Chrissaf's sketch.