Another air ride link Question?

[ironadict]

Just picked up 57 chevy stepside. Planning my rear set up. Was thinking bag over bar 4 link but after researching I started thinking " why are there no parallel 4 link bag over bar set ups?
I was thinking lower bar on inside of frame with bag over it. Upper bar on outside of frame. Then a watts link in back to keep it centered.
Is there something I'm not seeing in this set up that would be a reason not to do it?