Brent,
Good analogy there; it is a chicken/egg conundrum here. You should be able to order the dies and use them to form some preliminary 6x284 rounds. A simple guess on the size of bushing should get you pretty close. However, once the cases are in hand, measure the necks, and then follow Redding's advice on how to select the proper size bushing from that point. Some experimentation from there may see you refine that a bit further, but that's up to you.
I'll also depart from some of the other comments here, as well: I do use an expander ball, even with the bushing dies. I prefer Redding's carbide expander, and select a bushing that gives just the slightest kiss to the I.D. of the necks as the ball makes its passage. This may be due to the volume of reloading I do (I shoot HighPower competitively, and load in large batches), and someone who loads smaller quantities may not choose to go this route. I just want some contact, to assure me that the I.D.s (and thus the neck tensions) are the same throughout the entire lot. Again, you'll need to adapt your particular prectices to whatever YOUR needs are, not someone elses.
Hope that helps,
Kevin Thomas
Lapua USA