I used the stock F150 shackle as the double and a 6" as my leaf shackle. It works pretty good. I think you can use the Chevy shackles the same way. Only company I know of that sold a double shackle was Downey.
thats basically how i did mine but i cut the top off of a pair of motor mounts and welded them to the bottom of the frame so it didnt make a clanking sound
