I have factory lowers on my car with the ATR bar. My lowers are boxed, and I put the special steel spacers that came with the bar to keep the arms from collapsing before welding the boxing plates in. No way I would run the ATR bar on non boxed arms, or without those spacers.
FYI, although I don't have one on the GN, I helped put one on a G-body car (Monte SS). Like it says on the website, the BMR bar not work with the stock lower CA arms, but we also had to grind the heck out of the hump part that goes around the differential pumpkin on a Monte SS (Note that the Monte SS has a different rear than stock GN's. I think it has 7.5 or so inch ring gear as opposed to 8.5 like our cars..). It just didn't fit to our liking without rubbing the diff cover a great deal. We had another brand lower arm setup on there though (I think Hotchkiss), so it might have just been bad luck.
Billy