Note that I'm using the rules from the 'Shooting Phase' section for this, as that comes closest to what we're after (and takes place in the same phase). I would say either from the final position of the Reavers, or randomise. Logic for the former, is that the bladevane attacks are resolved -after- the unit has completed its move, and you take casualties from the closest model to the unit that inflicted the damage.
The alternative is to use Random Allocation on p.35, where (paraphrasing) it says if you can't work out the direction of an attack, randomly determine a model in the unit and that model counts as being the closest until the attack ends or it dies.
What I would do? Consult with the opponent before the game and come to an agreement, if an agreement cannot be made default to random allocation.