I would set the radius arm length so that the front driveshaft has the least amount of plunge as possible. This will cut down on suspension binding under power.
Look at how the Range Rovers and FJ-80's attach their front radius arms, far superior to the Ford design and much easier to fab.
Ideally the trac-bar and the drag link will be the same length. They will also be parallel as veiwed from the front and from the top with the vehicle at ride height and with the steering pointed straight ahead.
If they can't be the same length then you need to set them up so that the arc of each closely matches the other within the normal range of motion. Which means they won't be parallel as viewed from the front.