You'll not be able to use the stock shackle parts with the new shackles as the stock shackles have the pins pressed/welded into one side and the other side has a stepped down hole that matches with a shoulder on the pins. You might try using the stock shackle as is, but likely if OME supplied a longer shackle, it is because the longer shackle is needed for the springs. A shackle must be long enough to accomodate the change in length of the spring from full extension to full compression without having the shackle limit the travel. As I recall and earlier post, your shackles had some sort of anti-inversion tabs on them so that is probably a concern of OMEs, that when the spring is fully extended, the shackle may swing too far forward and invert or fold up onto the frame.

BEst bet would be to use their shackles and determine the angle change needed via shims. As I mention on my shackle page, you'll see about 1 degree of pinion angle tilt per extra inch of shackle length (stock is just short of 3.5" center-center). So you can compare this "rule of thumb" measurement with what you find on the angle finder and see if the measurements sound reasonable. Then its easy to make a shim to correct the angle at the pinion flange.