Steering box is up to you, FE FC have the longest column, FB EK are about 50mm Shorter and EJ EH are about 50mm shorter again IIRC. HD HR columns are too short to be comfortable, Whatever steering box you use, you will need to use the HD HR Pitman arm. The HD HR pitman arm will fit on all the steering boxes.
Keith
Pretty sure EJ EH HD HR are all the same length HD HR only has two mounting points for the steering box to subframe as EJ EH has 3 and the holes need a little work to align ....FC427......