The cylinders on a double row are counted from the top as 1(R)-2(F)-3(R)-4(F) alternating between the two rows, and so on around the engine. The firing order needs to alternate between the rows and therefore 1-3-5- and so on isn't possible. I think it is:
1-10-5-14-9-4-13-8-3-12-7-2-11-6, alternating even (front row) and odd (rear row) cylinders. I could be wrong though.