A little early, Joel....but I'll do what I can when the time comes!
Saw your front end pix and parts photos. Fun times ahead...
The GW (rear) lowers--assume we are talking the primo version with spherical bearing at the frame end and DelALum at the axle end--take all of the motion that resulted from deflection within the rubber bushings and flexing of the stamped arm and replaces that with the spherical bearing's ability to articulate "x-degrees" in each direction, sufficient for free axle movement in its normal limited range (by length of shocks--extended--and by bump stops--fully compressed) in a situation where one side is completely up and the other down. The Del-A-Lum bushing(s) in the axle-end of each arm has zero deflection, essentially. Properly installed, the lower arms alone will provide 90%of the work/force to locate the axle side to side, but when combined with the upper arms operating at opposing angles, the rear axle should be quite resistant to side motion, regardless of the type of bushings/bearings used in the upper arms. Look at it as a form of triangulation--thats about the best I can do to describe it.
Because of the way the GW lower arms rotate as the suspension moves, it is especially important to have good fasteners securing the rear sway bar to the arms.
This is not to cast doubt on the way the BMR upper arms you are speaking of work. If the link is adjustable, is a solid tube that cannot flex like the stock stamped part, and the bushings have limited flexibility (poly?) then I do see a problem with them. Based on a number of comments about the bushings coming out of them, I would be inclined to not use them unless I was doing "straight-line" work only--but very few of us limit our cars to that sort of driving.
Without spilling too many beans here, you are thinking in the right direction in your comments about having a DelALum in the "length" of the upper arm(s). Not exactly what it will be, but in the end it will seem to be a fairly simple (now why didn't I think of that!), even elegant design.
As for your last post, the concern you are expressing is what has been bothering me from the beginning with moving the axle back. The fixed "target" of the 2 frame mounting positions with 3/4" of additional control arm--or whatever the added length calculates out to for the angle that they run--would be to 2 new mounting points further out on each side of existing points on the frame bulkhead. So, when it comes time to put the bolt through the hole for the bushing at one end or the other of the upper arm, the only way to get the link and bushing to line up is by force, so you are starting out with the bushing side-loaded statically--NOT good, and part of the reason, I suspect, that the BMR uppers' bushings have been problematic. I have assumed that one solution would be to offset the link between the axle bushing and bulkhead bushing more or less as you describe, but no manufacturer has seen fit to do that yet, since the Impala SS and Caprice are the only cars this "axle relocation" is being performed on, to my knowledge, and they probably don't want to complicate their manufacturing by having a special part "just for this configuraton" even though it is fairly common in our community/vehicle population. Recognizing the problem is only half of the answer--getting someone to provide the fix is the other. Some of us with fabricating skills could cut and re-weld the link at one end or the other to correct this alignment problem--you too, George!
The new GW rear upper system will take care of this problem by its design, so no issue there. The system will work very well with stock rubber bushings (on the axle ears) for standard location axles. There will be an added bearing in place of the bushing(s) for the rear axle if the axle has been relocated, but the added bearing can also be used on standard location setups, too.
The only thing that has me a little "bugged" in the whole axle location business is just how important the actual angle of the upper control arms is to the overall side-to-side control and positioning of the axle in relation to the frame/body. From looking at it, I think the total angle spread between the 2 upper control arms is at or greater than 90 degrees. I have not done any measuring, nor have I determined what the change in angle is (it becomes less--a narrower angle--that I am certain) when the axle is moved back. If the stock arms are at exactly 90 degrees, then extended arms will result in a total angle of opposition of the 2 upper arms of less than 90 degrees. I'd have to go back and study my 7th grade geometry, but I am fairly certain that in a triangle, or in a triangulation situation, any angle other than "exactly" 90 degrees becomes unstable, so the whole question of how well the upper arms will maintain proper side-to-side location becomes a concern and occasion for further investigation. Maybe someone with a M.E. degree or with a CAD background can do a better job of evaluating how it works and why.
Are you still with me...George?