I think it comes down to cost in the end. In my opinion the more parallel the arms, the more articulation you'll have with less bind.
there is an instant bind as soon as you articulate radius arms. Most oem's use decent sized bushings at the axle to allow flex and not try to twist the mounts off the axle. He'd actually have more articulation with less bind if setup with them angled in.
Eric, they mounted them that way so as the axle articulates, the bushings and arms are not under a ton of stress while loaded down. the center of the axle is the pivot point (to an extent) as one side of the axle is forced up or down. with the links near the middle, the bushings can flex with the axle. if it were straight, the bushing would be in a more complex angle and would not be able to flex as far (closer you are to the center of the axle, the less its moving as it articulates). In your situation, this is not nearly as much of an issues as it seems.
i would still have some inward angle on them at first and see how it does. then play with moving them in or out and see how the truck reacts. as you can see in brets truck, the inboard mounting didnt make the truck overly squirrly or anything.