Steering bar lengths

I've gone and got myself 2 defender axles, and I think I'm going to cut the coiler bits off them and fit them to my series - load of threads on that, so I'm happy.

What I can't find is what length of steering bar I need to connect the series steering relay to the defender steering knuckle - I assume the defender one because the axle is wider?? Problem is that this axle hasn't got one.....
Will a disco one fit? - I imagine they'll be easier to source.