pretty sure you will find the short end of the spring goes to the front in order to compensate for spring wind up under acceleration, here is a link to the gm upfitter page with some frame info that shows axle center line in relation to the frame. maybe helpful?
https://www.gmupfitter.com/files/med...LD_STTruck.pdf
as far as the shackles go, the open end of the shackle needs to go up to fit the frame between the outside parts of the shackle, and the brace between the 2 side pieces goes towards the bottom rear side of the springs. the bolts go through from the inside out so the nut goes on the outside of the frame.
maybe this link will show better than my explanation on the shackles and the bolt.
https://www.youtube.com/watch?v=cdcS-AtMZSM