its something i have thought about for a while but just finally got around to building. Why the lower arm/mount? To stabilize the lower arm. With the stock shock removed and just the bag in place it becomes becomes unstable and twists. So it is basicly an "A" arm now with 2 mounting locations.
I have driven it and so far so good. When i didn't have the lower arm it twisted and it wouldn't even lift so it couldn't be moved. So I think i'm ok. Some people just put the bag on the lower arm and it works for some . I think it is a function of how bad of shape your crossmember bushing is in. But I don't want it to twist when my new bushing goes bad.
its pretty stable - its a 7.5" long bolt at the crossmember now. its a 9/16 grade 8 which is a slight bit bigger than the metric 10.9 that was there. I had to run a drill through the bushing to "clean it out" so the bolt would fit through.
oh - almost - i am still building trust in it now. my last air i beat the heck out of and it stayed together. i am still careful over things to listen and see hoe it goes. it has only been on there driving for about 4 days now.