At the risk of sounding like an idiot

[Get Down]

Can somebody clue me in. I'm looking to put 3" lowering blocks on the rear of my truck sounds easy enough. The shackles on the rear of my 49 3100 seem auful long. Should I leave them or replace them? If I replace them will that raise my rear axel also? Here is a picture to either help understand what I'm trying to say, or show my concern. I guess I'm asking if I should remove that rear shackle or just add the 3" lowering blocks and everything would be fine. If anybody care's to share pictures of their same set up I would appreciate it. Thank you