The stock shackles barely allow the spring eyes to clear the pivots. I'm not at home, so I can't give you an exact measurement, but they're pretty damned short- maybe 4" total length, 2.5" or 3" eye to eye?
Also, the stock shackles have a slight outward-facing flange running down them and no center brace bar (commonly backyard-welded between the two elongated shackle pieces for strength).
A stock '75 will ride very, very low. AMC jacked them up quite a bit with the suspension redesign of '76.