Yes, I am extremely jealous!! I hate my rear lift springs, and wish I could get rid of them and oakie-up install a rear shackle flip!!
The point is, the dude says he wants to tow up to 8,000 pounds...I don't know how well shackles in a shackle flipped set-up (no matter how thick or strong they are) will hold up to the side loading (sway) that will come with towing that much weight. A stock position shackle is less likely to be affected than a shackle flipped shackle in side loading. Best bet is to go with all spring.