[Jac Mac]

Its the two center cyl that crossover on each bank regardless of firing order, but as doug points out you should endeavour to make them fit in each collector in a circular firing order, this require a bit of pre planning to get the most suitable layout. So for:
13726548 its ..1-7-6-4..RHS set and 3-2-5-8...LHS set.
15426378 its ..1-4-6-7..RHS set and 5-2-3-8...LHS set

Now when you look hard at those numbers you find that the circular sequence works for either firing order as the only thing that really changes is the direction of rotation at the collector and that is not an issue .