Doctor Diff shafts

going to put a 8 3/4 rear axle underneath my Dart soon. So now the time has come to order me some new axle shafts.

But on the Doctor Diff site I have to make a choice between 3 options
30 spline carrier
35 spline spool (not Mark Williams)
Dana 60 35 spline carrier.

But which one do I need to order ?? Is the standard shaft always the 30 spline ??
Or do I only find out the correct one by pulling out the old shafts ??

gr Arnoud