why do you want to use the stock shackles? even if you move the hanger to use them, you would be limiting flex, the shorter shackle would bottom against the frame in a couple inches making the available flex of the spring useless. keep the longer shackles

KYOTA is right, it would be pointless for you to buy Marlin springs and then use your stock shackle.  You would not be able to use the flex they provide. 

They added leverage of a six inch shackle is not going to hurt you unless your frame is falling apart like mine was.  There are lots of guy's running 8" shackles and they have no problems except maybe dragging them on rocks now and then.