I have experience setting up Vega cross-steer. The usual procedure is to tack the box in first, since that's the most crowded and critical area, then fit your column to that by use of whatever connecting shaft lengths and u-joints are required.
Mount the box as horizontal as possible; the pitman arm should arc mostly horizontal. Also your drag link and tie rod should be as horizontal as possible, and as parallel with one another as possible (see illustrations). I add "as possible" on all those positions because cross-steer systems can tolerate some misalignment if it's necessary to solve clearance issues. But keep in mind that anything off horizontal becomes more acute at the extremes of steering system travel.
Set it up with the steering box in it's centered position and the front wheels centered (duh, right?).